I nominate Guinness.
No reason other than I feel like picking on little people today.
Q: Why does Guinness laugh when he's playing soccer?
A: Because the grass is tickling his scrotum.
Although I agree with and had a real good laugh at the joke at Guinness' expense I personally think that the reason that he laughs while he plays footy is that he is in a hysteric state of shock that he has actually been selected to take the pitch