Hello, anon, and thank you for the question.
This topic has been studied by researchers for years. There are three prevailing theories that I will relay to you now.
1. It keeps him on the ground.
You may notice in the gif above that Chris’ leg starts to rise as he laughs, possibly a precursor to his entire body undergoing a sort of lift off due to his joy. Chris then employs his upper body strength to force himself to obey the laws of gravity.
2. To check on his physique.
As you may be aware, anon, it takes a lot of hard work to maintain a superhero body. Chris is concerned that in the time he has spent sitting down, sans working out or eating, he has lost muscle mass. Understandably, he feels the need to make sure that he is still a specimen.
3. Object permanence.
Object permanence is a term applied to the understanding that an object still exists even when you cannot see it. Chris closes his eyes when he laughs, making him unable to see that he has not disappeared. By grabbing his left boob, Chris knows that he has not somehow ceased to exist.
I hope this helps.