Would you like this explained by reference to the Suttas?
(Awww… who am I kidding… its difficult for me to stop when it comes to quoting the Suttas! )
Consider this example from MN101
Suppose a man is in love with a woman, full of intense desire and lust. Then he sees her standing together with another man, chatting, giggling, and laughing.
What do you think, mendicants? Would that give rise to sorrow, lamentation, pain, sadness, and distress for him?”
“Yes, sir.
When the man realizes that he is experiencing Dukkha, he has realized the First Noble Truth.
Why is that? Because that man is in love with that woman, full of intense desire and lust.”
Then that man might think: ‘I’m in love with that woman, full of intense desire and lust. When I saw her standing together with another man, chatting, giggling, and laughing, it gave rise to sorrow, lamentation, pain, sadness, and distress for me.
This realization is the Second Noble Truth.
Why don’t I give up that desire and lust for that woman?’ So that’s what he did. Some time later he sees her again standing together with another man, chatting, giggling, and laughing.
What do you think, mendicants? Would that give rise to sorrow, lamentation, pain, sadness, and distress for him?”
“No, sir. Why is that? Because he no longer desires that woman.”
This is the Third Noble Truth.
From this admittedly coarse example, the sutta then progresses to analyzing the methodology behind the further development of the Path - thus fleshing out the Fourth Noble Truth.
(I’d like to point out that the 4 Noble Truths are Universal Truths. Hence they are equally valid for both the lover seeking relief from the Suffering felt due to his unrequited love as well as the ascetic seeking relief from the Suffering felt on account of unsatisfactory Sense experience.)