Dear SC community.
Once again, I need the help of the community.
Maybe someone could tell me where I could find such a sutta,
where the Buddha explains that if one focuses on a certain desire/object, it begins to increase in the mind and when one does not focus on the desire/object it begins to decrease in the mind.
I hope that you’re doing well and that you will have time to enforce the Buddhadhamma on a daily basis.
Whatever a mendicant frequently thinks about and considers becomes their heart’s inclination. If they often think about and consider sensual thoughts, they’ve given up the thought of renunciation to cultivate sensual thought. Their mind inclines to sensual thoughts. If they often think about and consider malicious thoughts … their mind inclines to malicious thoughts. If they often think about and consider cruel thoughts … their mind inclines to cruel thoughts. SuttaCentral
SN 46.51 the Ahara Sutta- Food is the classic description of how appropriate attention causes growth of beneficial qualities and starvation of undesirable qualities.
Glad you found the sutta you had in mind. There is also this series in the Book of the Ones in which the Buddha states the one thing - careless attention to what is attractive - that causes desire to arise & grow. He goes over what triggers each of the other hindrances, then states what prevents or causes each to abate (careful attention to what is unattractive causes desire to abate).