Is that a known behavioral symptom for anything? Or, just me being hypersensitive to a gut instinct?
It could be a bit of both.
First the normal side to things. If a person fancies another person, its likely they'll try to find ways to get chatting etc. (Think of the lad on the broadband advert.)
If they fall in love with that person people tend to take on aspects of their partner because its all part of what happens with that particular kind of madness, lol.
The other side of things - If people lack either a stable sense of self or lack some normal components of "adult personality" that can mean they can tend to mirror people.
Basically, I've known two people at different times in my life who seemed to have very shallow, almost pretend personalities. I never felt like I was truly seeing the real them.
I can't speak for everyone but I know people could see me that way. I'm not a pretend person underneath in any way, but while for most people their "surface selves" - the mannerisms, ways of speaking, external preferences and socialized environment they get from their parents literally become part of them, for me its more like a language that I speak, and I can learn to speak other languages too. Sometimes I like those other languages more, but nothing really "sticks" as a genuine part of me. What there is of me on the inside is solid. What there is on the outside is flexible.
Ironically I think sometimes its the opposite way round people see things. They can see people as having nothing on the inside when in reality the truth is much closer to having nothing on the outside, i.e. no emotionally internalized mode of interacting, not making external stuff which you just grow up around or become used to internal. Its not about having no feelings about anything at all, though there are areas where a person may have no specific feelings about anything.
In that sense, some of those things can be less "who you are" and more like putting on a T-Shirt in the morning. I can have preferances about some things, but its a lot more like deciding whether I want to paint my lounge red or green - those things are still my choices, not literally me.
imagine you're dropped down in a world where everyone has absolutely fixed and integrated emotional values about circles.
There are hundreds of different feelings you can have about circles, and everyone has them. If you don't, you're going to stick out like a sore thumb, and not in a good way.
You're expected to express those values because they're supposed to be an important part of how you feel about yourself and how you relate to others.
You're not seen as a complete person unless you express your strongest feelings about circles - people who don't are seen as immature, vacuous or empty shells.
- So you learn to make sure people don't notice you don't feel strongly about circles. To do this, you have to learn from how other people talk about circles. This involves mimicry.
I don't copy people's ideas or their clothing, or anything like that. I don't tend to take on ideas (whether from other people or random external sources) without understanding them to the point where they fit into a bigger picture of some sort in a way which makes sense to me and I don't try to pass off other people's ideas as my own in any kind of personal context.
I'm creative and perfectly capable of coming up with my own ideas - if I happen to share tastes with another person, that really is all it is. If I appear to "mirror" them past what's reasonably covered here, its worth assuming I fancy them.
I once remember someone trying to suggest I'd copied their taste in music - imo that was pretty egocentric of them, lol. Most people would say "cool, you like some of the same music as me!"
I also understand when you say shallow, almost pretend personalities, you mean what you see on the surface doesn't seem to go any deeper rather than "that they are a shallow person". I could see people seeing me that way in some respects, - someone once mistakenly asked here on these forums "if you didn't have strong emotions would you be shallow" but I don't think I do have particularly strong emotions and I don't see myself as a shallow person at all, though to me what makes me not shallow isn't mainly how I feel about details, interactions etc, its what I think, but also that there is a level of feeling about life and experiences in general attached to that on some level.