There are a few reasons why Alonso would want to leave, as far as I can tell. One was that stuff last year with Barry. Also, he probably fancies a move back home, maybe to raise his kid there, whatever. With the new era of scum at Real, he probably wants to be part of that too (although he won't be as remembered as if he won the League here). Also, it's possibly his last "big" transfer in terms of his age. Probably more, but I can't really be bothered as they're all obvious.
What I don't understand is that he is a Basque. So why would he be alright with joining the Franco Fan Club?
But meh. I hope he doesn't leave. "He's the man on the ball" as Reina aptly put it.
Oh the man is a midfield maestro,
And his passes are soooo delightful,
Everyone wants to know,
Alonso, Alonso, Alonso!