“Okay, so let’s say you meet a person who is leaving for college the next day. You spend an entire day with them talking. You are totally clicking with them, you feel a strong connection, and you’re really bonding. You stay up all night talking; you are both being honest and open with each other about your feelings, and you end up having sex.
The person leaves town the next day, as they told you they would. There are no hard feelings between either of you and you are left with a fond romantic memory. Maybe you’ll keep in touch, maybe you won’t.
Is there really anything so wrong with that?
Let me introduce a commentary which does not necessarily answer your question directly but does offer an alternative way of looking at why it becomes “good” to do what you are claiming is good.
Modern thinking suggests that personal pleasure and the personal gains and profits of the individual are the most important factors in the determination of actions. This mindset does not necessarily reflect the best way of acting, even though it has turned from “opinion” to “knowledge” in contemporary America. Such a thought process is no longer a philosophy – it is now the only way to live. The result of this thought process is the contemporary western world: one which claims to be enlightened on the surface but experiences great personal misery below the gilding, one which slaughters innumerable people with the technology which was supposed to ensure a greater peace, instead sowing strife, and hollowness of human affairs.
Rejecting extravagant indulgences and seeking to minimize desire is another alternative standard to live by. Surely the pre-Modern man thought of a heroic individual as one which rejected all desire and sought to serve the great City of humanity while exemplifying the virtues of prudence, justice, fortitude and wisdom. Austerity and a strict economy were considered virtues instead of failings. Material things were considered irrelevant and in flux, civic virtue and the pursuit of ideals replacing the pursuit of gold. As Heraclitus famously put it:
No man ever steps in the same river twice, for it’s not the same river and he’s not the same man.
The question becomes: why should we pursue or find value with the sort of arrangement you have described? What is the standard to which your actions are chosen and is that standard one which, if adopted by all people, would result in a functional long term society?