Gender dysphoria results from someone's displayed gender/appearance not matching with what they feel it should be. Eg. if a young person realises they're transgender while going through puberty, they're not gonna sit around and wait while their body changes into something that they dislike. They haven't as much chosen to be transgender as they have chosen to act on it.
From what I've seen with trans people I've met they seem a lot happier after their transition, only a tiny number of people choose to have extensive gender-affirming surgery or for some, to detransition. Granted I live in a fairly progressive area with lots of support and acceptance for these people so their happiness will depend a lot on that.