When Snape is around and Harry is in danger, Snape will protect him. Remember when Draco conjured up a snake during the duel? Snape attempted to get rid of it before Gilderoy Lockhart stepped in and blew the snake into the air. Then when Lupin turned into a Werewolf, Snape stepped in front of the students to protect them. Remember the scene from movie 1 when Snape didn't trust Querle and he had him pinned up against the wall, telling him to decide where his loyalties lie. The only reason Harry saw that was because he was underneath his invisibility cloak. It was a look into the real Snape. Dumbledore trusts him for a reason.