You can be "surprised" in both positive and negative ways. When you are surprised in a positive way, you can say that you were "pleasantly surprised":

A: How was the art show?

B: You know, I thought it would be boring, but I was pleasantly surprised.

The word "pleasant" means "good". So why do people say "pleasantly surprised" instead of "positively surprised" or "surprised in a good way"? There's no real reason, except that these words usually go together. This is an example of collocation.