Usually he is pretty friendly, and while he sometimes accidentally pokes someone with his claws, he rarely does so on purpose. But Dave has been on edge lately. Back during his days growing up on the mean streets of Iowa, he always had to be on guard. Even in his quiet domestic life, he has retained some of his past disposition, always ready for a fight. Here, you can see Dave is biting Jack. Dave never does any real damage, but it is just one of the realities of living with Dave.
In Dave's defense, Jack is usually being a jerkface when Dave decides to bite him. Jack, though much smaller than Dave, can still be a pesky guy. He might have been trying to steal Dave's napping location. In that case, Dave might have been perfectly justified in biting Jack. Naps are important.