I have a 5 year old Labrador. He's very friendly- never bites, growls, or even barks really. But he likes to run after and people when they walk by! Even if I'm playing catch with him he'll drop everything and go bounding after them. He never does it violently, he just approaches them and follows them. I take him on regular walks and play catch with him but he just won't stop! What can I do about this?!