To be honest, I feel the same way. It's hard to say why. However, it could be that I don't understand it.

I think someone's acting like a different person when they pretend to like something they don't, or are interested in things they usually dislike. Although, when they discuss something even though they don't like it, or give something a try they usually don't like, that doesn't seem like they're acting like a different person. That's seems different. I think it's when they're changing more fundamental parts of their personality to fit in that seem to give me the more negative feelings. I can usually only tell if I've been around the person long enough, though, and by then I probably care more about them.

When I think over it, it might be that it makes me wonder: if they act a certain way with a certain person, do they act differently with me? Also, maybe they aren't comfortable putting on a mask, and feel the need to act very different because they're so afraid no one will like them, which is a little depressing. I think it's just because I'd want them to be more open with me. It does seem to mean that they're lying to some extent as well. Maybe this is harsh, sorry. It's what goes through my head when I think about it.

I don't feel any negative feelings when someone acts a little different depending on who they're around. I think it's okay to have a more polite and professional side vs a more friendly, open side, but I don't think that means you're acting like a different person.