Why would someone lie about it? How does it affect your life? Can't you trust that they have thought a lot about this before identifying this way? Now there is nothing wrong with being curious and asking them questions, but don't do it in a way that comes off as treating them like they are stupid or as accusing them.
It's one of the worst feelings to know that the thing you worked for overt three years to overcome is trying to be pushed back by someone thinking they know who i really am..
Because they are in doubt over their own identity; or haven't truly quesitoned/explored their own identity enough; or feel threatened/vulnerable in/scared of their own identity or by the identity of others.
Yeah, I dunno. Speaking from the perspective of having family who does this, think it doesn't always cone from a place of malice but from one of attempting to help, however misguided. In my case, they believe I'm having an identity crisis and am mistakenly becoming someone else. In a world where cishet's the default and any deviation's gotta have the ten page backstory with it, coming out, especially as trans when no one else has seen a history of gender dysphoria, is one hell of a mess.
I think it is often times it is a genuine (though perhaps misguided) desire to help someone, but filtered through the lens of one's own experience. For example, someone who themselves struggled a lot with accepting who they were, and who sees someone else having what they perceive to be the same struggles, might assume that the experiences, feelings, and outcomes are the same. It can also come from an insecurity, along with an "I know what you're thinking and I know better" sort of thing. In probably 90% or more of cases I don't think there's any ill intent, but it can certainly interfere with a person's own process.
When my older sister found out I was gay she assumed I was doing it for attention and trying to cause problems. Ever since I came out to everyone our relationship has been sour ever since and we don't even speak to each other anymore. She refuses to believe I am gay and in every argument she has had with me has not even said that I am gay. She is too scared of the truth.
Straight people never have had to experience exploring their sexual identity like we have. So they don't know how much effort and time we've put into determining ours. We've just experienced something that they can't even comprehend. And for a lot of religious folk, they don't want the reality to be true that we're queer. So they try to deny it.
Insecurity, mate. Lots of projection on their own behalf. People who are insecure in themselves always react badly towards those who are comfortable with who they are.
Maybe because they already have a perception of your identity and it's like all their life they've thought the sky was green and now you're telling them it's blue and they've just been wearing tinted glasses. And they're like, WTF, no I'm not!
Used to happen a lot when I would tell people the word 'asexual' all these questions about temptations, hornyness, masturbation, blablabla But when I'd say 'bi' or 'gay' they didn't give me the questionnaire. I think it's just ignorance, because terms like "asexual" or "pansexual" are not generally used, so people just ask stuff out of curiosity, but they do it in a way of comparing it with the sexualities that they already know of, and that makes them seem like they're trying to prove you're something else, and it's actually rude.
