Well let's break down the actual real Grail War participants and see why they're fighting....
Assassin (Fake): Basically, a bouncer. Hung out a lot? Liked to fight, I guess?
Assassin's (Fake) Master: Caster.
Assassin (Real): Consummate professional who does job.
Assassin's (Real) Master: He's just doing it because that's what he wrote in his dayplanner for this century and he doesn't want to admit he has <a href='http://www.penny-arcade.com/comic/2008/9/5/' target='_blank' rel='nofollow'>Alzheimer's.</a>
Archer: Wants to paradox himself out of existence.
Archer's Master: Actually wants to win, sorta; it's what's expected of her.
Berserker: Protection complex? His motives aren't actually explored, on account of having no dialogue that can't be rendered (Roaring Sounds), and no internal monologue.
Berserker's Master: For the lultz. Also a brocon with daddy issues.
Caster: Wants to play house (is terrible at it).
Caster's Master #1: Died really early, actively sabotaged own chances of winning.
Caster's Master #2: Convinced to play house (had no real objections).
Lancer: Wants to fight? Acts like it's a block party for the Throne of Heros.
Lancer's Master #1: Wants to hang out with Lancer.
Lancer's Master #2: His origin is "trolling", and he's true to his origin.
Saber: Actually wants to win Grail.
Saber' Master: In WAY over his head. Mostly wants to prevent casualties.
Rider: Siscon, with regrets. Might want to fix life with grail.
Rider's Master (Fake): Wants to win grail, but really, wants to prove himself not inferior.
Rider's Master (Real): Some flavor of Yandere.
Gilgamesh: Wants world to be awesome, according to his narrow concept of awesome, and wants to use Grail to do that.
Gilgamesh's master: That guy with the origin of "trolling" again, the cheater.
. . .
So by my count, of the like, nine Servants who show up, only two of them actually explicitly want anything out of the grail, one or two more might actually have some idea of what to do with it, and the remainder would basically be like, "huh, so now what?" Except Archer, who is here literally to sabotage himself. Because they were summoned, not by who wants it most, but by completely different criteria.
So it's really the masters who are in it for the sake of winning the grail, but from the three big houses that drive the war, the masters are all doing it because of obligation to previous generations, not because they have any flippin' clue of what they'd do with the thing, and the remainder of the masters don't even seem to really understand what the grail does in the first place. Except Kotomine, but again, his origin is trolling, so he's really just being true to that.
The Grail is a MacGuffin, it's just here to move the plot along, and the only ones that aren't willing to tacitly accept that as the <s>excuse</s> reason to have a bunch of punching contests is Saber, the party-pooper. Also that asshole Gilgamesh.