Great quote from a conversation between two of my favorite writers, Joss Whedon and Neil Gaiman --
"I saw a lovely analogy recently. Somebody said that writers are like otters. And otters are really hard to train. Dolphins are easy to train. They do a trick, you give them a fish, they do the trick again, you give them a fish. They will keep doing that trick until the end of time.
Otters, if they do a trick and you give them a fish, the next time they'll do a better trick or a different trick because they'd already done that one. And writers tend to be otters. Most of us get pretty bored doing the same trick. We've done it, so let's do something different."
~ Neil Gaiman to Joss Whedon, on why they both ended their franchises while Sandman and Buffy were still popular.