Conversations like this make me wonder what is the nature of mental issues. Why do we decide that if a person is born completely as one gender but somehow has

Conversations like this make me wonder what is the nature of mental issues. Why do we decide that if a person is born completely as one gender but somehow has this feeling that they are actually the opposite one that their body just chose the wrong genitals while if someone is born with a mind predisposed toward murder they are insane. What is the criteria that decides it? Obviously feeling like a man when born a woman doesn't hurt anyone, but does washing my hands "too much" hurt anyone? Personally I just consider you all filthy.