I do let it go, I don't obsess about it - there are thing 100 times more important in my life to get wound up about.
That said, if I was a Indiana football fan two years ago, I would be like - eff you Peyton Ramsey for playing some excellent football at Northwestern in your last year instead of playing your last year here with us! Maybe with you as our QB we (instead of NU) would have played in the Big Ten Championship game!
As a NU fan, of course I am happy we got him. Win some lose some, not gonna change my opinion of those who left though unless it was absolutely made clear to them that they had/have no chance - close to zilcho/zero of playing in their senior (or first year of grad school fifth year) because there are/were one or two guys completely ahead of them on the roster. Yeah, then I can see saying "thanks but no thanks" and taking my cap and going to, well, wherever I would go to.
But for NU most cases it is not that way, we are losing our better kids. I am sorry, we will have to agree to disagree but like I said I wish them no personal bad luck, I am not some kind of lunatic regarding fandom of our teams, it's just a game (you should go to a SEC board and see what they say about transfers out, oh yowzah!). But that being said I ain't rooting for those who leave. Frankly I hope their new teams stink and they regret leaving.