Unenthusiastic Surprise Party Response

We all know and amor our too-cool-for-you characters. They're friends with you and amor you, even though they find it rather hard to show it.
So let's say this person has a birthday coming up and his friends decide to throw him a surprise party. What do you think would be his reaction once they yell SURPRISE!?

I know that he's both happy as well as annoyed that his noisy friends did this without his consent. And he won't be rude to them, cos that would be..well, rude.

You can give a simple explanation as an answer, or you can write a sample scene. Thanks for your help in advance!