[Raynor]

Originally Posted by alatar

But maybe this explains the elves, as they seem to resist change, and when the environment is not to their liking, they fade away into the West, unlike Men and Ents, who are more adaptable.

I think quite the contrary is true; the elves are more adapted to 'living' than all other incarnates, given that their doom is to abide (in incarnate form, desireably) till the end of Ea.

Originally Posted by Of Aman and mortal Men, Myths Transformed

The nature of an Elvish fea was to endure the world to the end, and an Elvish hroa was also longeval by nature

True enough, there is fading, due to the marring of Melkor (same source):

For they hold that the failure of their hroar to endure in vitality unwearied as long as their fear - a process which was not observed until the later ages - is due to the Marring of Arda, and comes of the Shadow, and of the taint of Melkor that touches all the matter (or hroa) of Arda, if not indeed of all Ea

and due to the consumation of hroa by fea:

Originally Posted by Aelfwine's preamble, Laws and customs of the Eldar, Later Quenta Silmarillion, HoME XI

Moreover their body and spirit are not separated but coherent. As the weight of the years, with all their changes of desire and thought, gathers upon the spirit of the Eldar, so do the impulses and moods of their bodies change. This the Eldar mean when they speak of their spirits consuming them; and they say that ere Arda ends all the Eldalie on earth will have become as spirits invisible to mortal eyes, unless they will to be seen by some among Men into whose minds they may enter directly

Among other reasons of the fading, we can count the doom of Men to rule Arda and the curse of the noldor:

Originally Posted by Of the flight of the Noldor, Silmarillion

And those that endure in Middle-earth and come not to Mandos shall grow weary of the world as with a great burden, and shall wane, and become as shadows of regret before the younger race that cometh after. The Valar have spoken

(however, I think we can discount this at least in part, given that it is stated that curse was laid to rest after the war of wrath)