Mimetics
"Man is the creature who does not know what to desire, and he turns to others in order to make up his mind. We desire what others desire because we imitate their desires." - René Girard, according to Wikipedia
This is the thesis of mimetic theory. There's a popular nonfiction book about it, but I started it and really didn't like it. And I haven't studied the philosophical texts behind the theory. So in fact all I know is the thesis! But it feels like such a useful truth that it's honestly enough on its own.
We desire what others desire. This feels so true, in my experience. I don't know if I have an above-average mimetic weakness, but I frequently notice that exposure to something makes me desire it. If I hear a friend play piano, suddenly I really want to learn piano. Watch a successful standup comedian, and I want to do standup. The desire doesn't well up from within me, it manifests suddenly in response to my environment.
This phenomenon dovetails with other axioms that I believe. You are the average of the five people you are closest to. This is in part because you'll come to desire what they desire! If you surround yourself with ambitious entrepreneurs, you'll very likely find yourself wanting to start a business.
There's also socially induced lifestyle creep. You probably don't wake up one day and suddenly want to have an alligator-skin wallet. That's weird, and the poor alligator. But if you see everyone around you carrying the same, your lizard brain (lol) cleverly deduces that they must have a good reason to want it, and you may start to want it too.
I have no idea how much of this aligns with Girard's writings, but nonetheless I find "mimetic desire" a useful label. It brings my attention to how sensitive we are to others' desires, and to the tendency to unconsciously manufacture our desires based on those of others. And this is manipulable! If we acknowledge that we'll come to want what the people around us want, we can try to spend as much time as possible with people that want the things we want to want.
As for how we come to want to want something...