As
@twoface262 mentioned, porn can contribute to what you're describing.
Porn-induced erectile dysfunction (PIED) is caused by excessive porn consumption, where you your brain becomes addicted to the dopamine spikes of seeing a new woman in a new position with every swipe of your fingertips. The availability and convenience of internet pornography makes this quite common with younger men who grew up with internet porn since their early teens (during the formative stages of their brain and behaviour, and almost always before their first sexual experience). Unsurprisingly, this is why PIED is not as common in men in their late 20s and older (which often is the demographic who hastily dismiss existence of PIED).
The novelty of a new partner can wear off quickly if your brain is accustomed to being primarily stimulated from porn. Sex with one girl will become vanilla in comparison to what your brain is used to experiencing in porn (your dopamine receptors are stimulated less).
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Even outside of porn, the novelty can wear off with a partner rapidly if you interact too frequently with them. For some guys once a day is fine, others once or twice a month is good enough. For me personally, once or twice a week is just dandy, even in a relationship. We're all different.
For a lot of guys it's just about the conquest. Just being able to finally sleep with a girl is enough to make you lose interest/attraction for her, especially if you bed her early and have nothing in common besides sex. I sometimes crave ice cream spontaneously or if I haven't had it for awhile, and once I indulge I am good for awhile. I lose interest in it temporarily. Sometimes I come across an ice cream that is so good it might be take me a week straight of indulging before I finally get sick of it.
Not sure if the analogy fits; I'm just honestly craving ice cream now.