The strongest feelings I've had in my life are when love has gone right and when love has gone bad. And I think strong feelings make good songs.
I'm not saying you have to be totally despondent or anything, but... in New York, it's cold sometimes; it rains sometimes; even if everything in your life is great, bad weather can set the mood. You can write songs in New York because it's not always perfect. To write a good song, things can't be perfect.
