I don't understand transness and would like some answers.

I don't wanna come off as hateful or transphobic, first of all, these are genuine questions and misunderstandings I have.

So, why is wanting to be the other gender not considered a delusion? I mean, if you say you're the other gender, when you're physically not, then why is it considered valid? You wouldn't affirm someone identifying as a different species, for example. I'm not trying to say psychology is wrong, again, just want an answer.

I personally can't wrap my head around how someone could be so uncomfortable with themselves to the point where they would prefer to be referred to with different pronouns that deviate from their sex. And what causes it, exactly? Is it from birth, or are there environmental influences? Can someone be influenced to be trans?

Again PLEASE don't villainize me for this! I don't wanna seem hateful. I'm just curious.