I've often wondered - what is a joke, and what makes it funny? Or what makes a funny situation funny, or not? Does there need to be surprise in it, or shock?
One thing I hugely admire in your comedy is that you're not cruel. You get mocked sometimes for writing sunny benign, "British" comedies. I love them (esp 4Weddings and Blackadder). Actors must love being sent a Richard Curtis script. Do your scripts go through many revisions before they reach the screen? Or does what you write usually work first time??
Finally, have you written for theatre? I'd love to see a play by you. I don't think I ever have.