And, of course, the other reason is to make him smile just for the sake of seeing him smile! I have a 'special' little ditty I made up for him that I've been singing to him since he was born. Each and every time I sing it to him he smiles. It's one of the best things ever!
My mom (first-time grandma) also has a song she made up for him that has been repeated quite frequently, and he has the same reaction. (However, the rest of the family has gotten tired of hearing it... but she doesn't care! :)
I may not sing in public ever, but at least I can sing for my favorite audience and get rewarded with love.