because of how they feel inside
say a guy is super into girly shit. They want to wear dresses and dye their hair and whatnot. Under the pretense that the guy in question is above the age of twelve and can actually develop opinions, then it makes sense for them to have want to have been a girl.
on the flip side, girl is super into sports and boy scouts and having a day with the boys. it's fine if they want to say they're the other gender because 99.9% of the time it affects no one but themselves.