Dear Miss Snark,
I'm a very good writer -- and if you don't believe me, I've got signed notes from several of your colleagues to prove it.
Seriously, I just received another handwritten "you're good, you're a good writer, but this book isn't for us, and I personally wish you great luck with it" rejection.
If it's as good as they say, why won't they take it on? Any tips for when good writing doesn't seem to trump after all?
Query other agents.
Not every good book is right for every agent.
Just recently I got a query with blurbs from established authors I knew and liked a LOT. The book sucked. Not just sort of, but REALLY sucked. Do I think those writers were wrong? Yes. Do they think I'm wrong? Yes. Solution: find another agent.
Don't fret about no.
Get to yes.