Everyone says "autism", but that alone doesn't explain it. It's best to go in detail rather than citing the correlation between autism and trannies every time.
>autogynephilia: can't get gf, becomes the gf, etc etc we've all heard it
>grass-is-greenerism: thinks his life would have been better overall if he were born a girl, likely induced by one too many "cute girls doing cute things" series
>exploitation of progressive stack: trades 3 of progressivism's most reviled traits (straight cis man) for 3 of their most worshipped ones (lesbian trans woman); some go a step further and claim to be jewish since they look like an A Wyatt Mann drawing
>"get out of male free": related to above; realizes that if he were a woman then feminists' blanket statements about men and masculinity wouldn't apply to him anymore
>alternatively, he might think that since he's nothing like feminists' violent, rapist, misogynistic, hyper-macho strawman, he must not actually be male
>girl-on-the-internet effect: sees the attention, whiteknighting, and free shit that women get in male-dominated circles, decides he wants in on that
>plus the simple fact that his friends and coworkers enable this shit; if he lived and/or worked in someplace more conservative, either the idea of transitioning would never cross his mind or he'd look down on it with derision/disgust