fun-shark

n.
A person with an exceptionally active sense of enjoyment, amusement, and/or pleasure.

Group walks by fountain.
Fun-shark: Hey guys, let's go run through the fountain.
Girl 1: Umnmm, no.
Fun-shark: Suit yourself (runs to fountain)
Girl 1 to Girl 2: Does he always get excited like this?
Girl 2: Yea, all the time. He's a real fun-shark.