14 month old obsessed with his hand?

My youngest son has the weirdest obsession with his left hand. Starting at about 5 months old, he began to just hold his hand out and stare at it. And I know that is normal because my almost 3 year old did it as well.

But it's at the point where he is almost obsessed with it. At least a couple of times an hour, he will hold it out and just look at it. And the past couple of weeks, he gets really, really upset when I am putting his shirts on and put that arm through the sleeve. He's a really laid back kid. Never causes any trouble, doesn't throw tantrums and he doesn't get even half as upset about anything else as he does when I put his sleeve on. And he won't tolerate having his sleeve down either. If I don't roll his left sleeve up to his elbow, he will just pull at the sleeve until he gets his arm completely out of the sleeve. Has anyone else dealt with something like this? I'm planning on asking his doctor about it at his appointment next month.