Why does my friend do this?

Sometimes whenever he is mad or thinking very hard about something he puts on this mad face and doesn't notice anything around him even when someone talks to him and he taps each of his fingers onto his thumb in order. Index to thumb, middle to thumb, ring to thumb, pinkie to thumb, ring to thumb, middle to thumb, index to thumb and so on. It looks really creepy. Then when he's all finished thinking or stop being mad, he would stretch and put on this really happy face and start laughing when he notices someone looking.