There's exactly one motivation for all trans people who transition, and it's that we can't feel comfortable and OK in ourselves if we don't. You hear a lot of cissexual (ie. not trans) people flapping their jaws about how we Shouldn't Be Allowed to transition because it's obviously Bad For Us and will make us Lonely And Miserable Forever - well, that's just another form of prejudice really. Denying us the right to make our own choices, or painting us as irresponsible/crazy/naive if we choose to get the help we need.
There's no magic wand that solves all of anybody's problems at one fell swoop, and transition's no different in that respect. Can it create more problems for a person? Yes, absolutely. Are more of the people who have a lot of trouble after transition MtF than FtM? Also yes. But does it take away a huge, overwhelming, inarticulable source of pain from your everyday life?
Also yes.
Besides which, how many trans people have you met who passed perfectly, and who therefore you didn't have any idea were trans? Or to put it another way - how many of the perfectly normal people you've met might have been trans? I find it very funny the way cis folk think they can always spot a transsexual.