He's signed up because he was offered. We can safely say that he's not crying at night and not dreaming 24/7 about winning the belt back. The guy was talking of retirement, superfights, Roy Jones, etc more times than I can count. Never heard of him dreaming to retire with the gold around his waist. Even after his loss, he kept talking of superfights, hiring Chuck Norris... it's like Shogun hiring Freddie Roach and getting his ass kicked by Sonnen. Silva will be embarrassed just like that. He doesn't have Cain's mentality and hunger for vengeance. He's been the best for too long and doesn't have anything to prove.
Yeah I'm sure Silva signed right up for an immediate title fight just for shits and giggles. Of course it does'nt mean anything, it's only one of the biggest fights in his career, win or lose.