At the end of X-2, Tidus comes back because the Fayth, as thanks to Yuna, are willing to dream for one lifetime longer to reform Tidus's body so he and Yuna can have a life together.
There's also some more stuff from the X-2.5 novel, where there's another type of summoning made from the summoner's imagination, and Tidus is revealed to actually be one of these formed from Yuna's memory of him and he could think himself out of existence if he ever become self-aware of this and has an existential crisis, kind of like what happened to Alucard in Hellsing. But that's all a bit silly and trying to set up stuff for a X-3 that will probably not happen because it's a bad idea, so take that as you will.