Similar to the Abarbanel brought by @jake, Rashi says that Yonah figured that if the people of Ninveh would listen to G-d's word and repent, it would make the Jewish people look that much worse for not repenting. He therefore tried to get out of delivering the message to them.

Similar to the Abarbanel brought by @jake, Rashi says that Yonah figured that if the people of Ninveh would listen to G-d's word and repent, it would make the Jewish people look that much worse for not repenting. He therefore tried to get out of delivering the message to them.