If I chose what I, as you put it 'already am', then it would mean living with chronic depression and suicidal thoughts. You say that there's no biological mandate for gender, but that's not really true, it could very well turn out that there is, it's just a very complicated thing to study. Maybe you're right and self love can be found without transitioning, but where are these people who've been able to repress this?
I'll tell you - they're coming out now in their 50s, 60s, 70s and even 80s having been forced to live a lie that entire time.
Some people are literally born a chimera of two sexes, we know this because there are quite frequently obvious signs, men who grow breasts, women with testes, but what's to stop it from affecting a persons brain as well? It's possible to be born with a male brain in a female body or the other way around.
Genetics is messy and sometimes you get strange results, but to call that a disease? You'd be calling everyone with detached earlobes diseased and measuring skull shape before long. There is no such thing as genetic purity.