In a Sutta, it is said that even high and divine meditations, being produced by choices and intentions, is of the nature to pass away, it is liable to end. This seems to be why the various heavenly realms are also not a real escape and have to decay as well.
Why is it so? Why is it, that what is conditioned, has to fall apart? The core stream winning insight of “Everything that has a beginning has an end”, why is that so? It seems so obvious on an intuitive level but I’m looking for a deeper understanding. Are there any Suttas or reasources that can help me? I was looking more from the angle of heavenly realms and existence there. There are many people who claim their god is uncreated and cannot fall apart, but that “eternal existence” always felt like a fantasy to me, and I can’t quite think why. I suppose something deep inside us tells us that all things have to perish. But no one’s the better teacher than the Buddha himself. So if any of you could point me in the right direction, it would be great! Thanks!
Interesting question. 
I’m not sure the Agamas really answer this particular “why”, even though they explain that it’s due to Ignorance. They describe conditioned things as arising, changing and ceasing, but I don’t know of a passage that argues out why conditionality necessarily entails impermanence.
Nāgārjuna, a few centuries later, does get into this. Although, rather than explaining “Why it is so”, he explains “why it couldn’t have been any other way”. 
Suppose something had a genuinely permanent nature of its own—svabhāva. If that supposedly permanent nature depended on causes and conditions, then it wouldn’t really be entirely “its own”. So something possessing svabhāva would have to be independent of conditions.
But then it becomes difficult to explain how it could change at all.
If something else can affect it, alter it, sustain it, destroy it, etc., then its state depends on something else. And if it really cannot be affected by anything else, then causal interaction becomes difficult to make sense of.
This is also a problem for the idea of an absolutely unconditioned being. You can posit an eternal, independent being, but once it creates, intervenes, responds, chooses, produces effects, etc., it is worth asking what exactly “unconditioned” still means there.
Nāgārjuna’s point, as I understand it, is basically that conditionality and fixed intrinsic nature don’t fit together.
A flame is an easy example. It depends on fuel, oxygen, and so on. Its continuing to exist is itself conditional. There isn’t a separate permanent “flame” underneath those conditions.
So perhaps:
“Whatever is subject to arising is subject to cessation”
Can be read more like: if something can arise through conditions, then it cannot have an independent nature which guarantees that it remains unchanged regardless of those conditions.
This is close to why Nāgārjuna connects dependent arising with emptiness:
“Whatever is dependently arisen, that we call emptiness.”
MMK 24.18
“Empty” here meaning empty of an independent, self-existing nature.
The Agamic answer seems more pedagogical: it explains that this conditioned experience is due to Ignorance, without necessarily explaining the “why”. Nāgārjuna gives a more explicit philosophical argument for why dependence and permanence don’t sit very well together.
Anyway, Nāgārjuna is dense, and I’m by no means an expert. But I hope I gave enough breadcrumbs to spark an interest and point in the direction to investigate why experiencing itself can not be anything other than unstable / anitya.
Well, I think if something exists because of a condition, it can only exist as long as that condition lasts. Once the condition falls away, the thing has to cease.
So now go one step back: the condition too depends on another condition, which in turn depends on yet another condition … and so forth. So they will all change and cease when their turn comes.
Everything that has a beginning begins because its condition is in place. Something that has no beginning (and therefore doesn’t have to cease) would have to exist from the beginning of times, or whatever that point may be called, and be independent of any condition. But it’s probably hard to discern such a point in time, or such a thing independent of conditions …
I believe ignorance is the reason why we keep beginning new things. Once we understand that they all have to cease we’ll probably stop beginning.
Now, that “all that is subject to arising is subject to cessation.” isn’t obvious at all since the phrase describes arising of the eye of Dhamma, so puthujjana has no such knowledge at all.
Impermanence in Dhamma has much more and profound meaning. So in order that you can see impermanence for yourself, first you must understand that you don’t understand what perception of impermanence is. Impermanence isn’t taught so you just accept it: yes All that is subject to arising is subject to cessation. But rather that you see it for yourself.
One who sees eye as impermanent has a right view. But if you ask Muslim whether eye is impermanent, he would find question rather strange, but then answered it, that certainly it is impermanent.
But impermanence of the eye, or the body in Dhamma is synonymous with anatta, so one who sees the eye as impermanent understand that interpretation of experience in terms: “I see, this is my eye” is wrong and depends on ignorance.
i
As far as I can understand, you seem to be asking why there is change? 
Here is how I understand it.
The suttas are clear that change is a fundamental quality (AN 10.29). Change and impermanence go hand in hand. To understand the reason for this, one needs to investigate and understand the fundamental law of idapaccayata- the basis of dependent origination.
SN12.37
When this exists, that comes to be; with the arising of this, that arises. When this does not exist, that does not come to be; with the cessation of this, that ceases.
So, what has arisen is a manifestation, it is not a uniquely independent Thing. It is a phenomenon that arises on the basis of subparts.. it has underlying causes/ conditions. This applies to any thing at all that you might take under consideration as an object of experience.
Because what has arisen is dependent on those subparts/ causes, it is manifest only till the time those subparts/ causes remain in interaction. If any of the subparts/ causes cease, what is currently manifest will no longer remain.. it will cease… and a fresh manifestation will arise, based on the subparts/ causes that exist then.
Hence what has arisen, changes.. it ceases.. something new arises.. ceases.. another new thing arises.. ceases.. again a new thing.. ceases.. change occurs ceaselessly… ever more complicatedly… eons of the All expanding.. eons of the All contracting. Its a fractal law, true at all scales.
At no point does any phenomenon which is currently manifest ever acquire any intrinsic ‘Essence’ .. its always dependent on causes, hence it will always tend to become otherwise - at a slower or faster rate (AN 7.66).
Why do the subparts/ causes cease? Its because any individual subpart/ cause is a manifest phenomenon in its own right. It too is dependent on causes! And every phenomenon-while manifest- can serve as a cause for another phenomenon.
Breaking it down further and further.. a manifest phenomenon arises on the basis of causes, each cause is a manifest phenomenon dependent on sub causes…so on and so forth… its causal conditionality all the way down..and all the way up!
No first point/ original cause/ basic intrinsic essence can ever be found.
So what is actually going on? It is an endlessly changing pattern of manifest phenomena… none of which have any intrinsic Essence or staying power.
What we take to be unchanging Things are actually interdependent Processes seen in a snapshot of time. And the nature of Process is to change and become otherwise.
We do not live in a world of Things and Fixed Substances. We arise within a Processual world of flowing events.
This is why everything changes. It is Anicca. How could that which changes in and of itself ever be satisfactory? It is Dukkha to attach to such phenomena. How could such a processual manifestation of causes and conditions ever have any independent Essence - any atta which could be the controller/ be controlled? It is Anatta.
*A Bonus which you may or may not understand*
At most basic level, there are two interdependent causes. The Buddha describing the experential world spoke of Namma/Rupa and Consciousness (SN 12.65). Physicists describing the physical world speak of Space/Time and Energy. Each depends on the other like two sheaves of reeds (or a tripod if you prefer) resting on each other. When either is pulled away, it all collapses into itself.
Ultimately, there is Nothing. Singularity. Nibbana.
(Its beyond all description because with the collapse of the entire framework, there is nothing by which it may be described.)