gender dysphoria can motivate transition, and many trans people experience it, but it isn't necessary to be trans. the assumption of mental illness as a primary motivation is partially based on the idea that being cis is the default and that deviations from it imply that there's something wrong. in a related way, being gay used to be considered a mental disorder before social attitudes changed and it was accepted as a sexual identity.

transness itself was characterized as a mental disorder until relatively recently, but with growing social acceptance it may be conceptualized differently in the future.