Horton is a pretty persistent guy throughout the book. Even when everyone's calling him crazy and trying to destroy his new friends, he doesn't give up. Anyone remember how many clovers he looks through to find the Whos? Three million.
There seems to be a moral in here: don't back down when you believe that what you're doing is right. Horton has a whole lot of conviction, which helps him as he stands up to all those mean animals. And ultimately, he saves the day.