After my cat died, my son's cat rather adopted me. When I come back after an absence (say, after work), he will run at me with his tail straight up and then when he gets close to me he shivers just the top third of his tail. He then usually runs past me, although he now sometimes comes back if I call out to him. He's more liable to get on my lap now that he is neutered. My vet said this was some sort of love thing.