1. Sebastian is not a nice person, though he is sometimes amused by pretending.

2. He cares about Ciel. He never intends to admit this, and truly doesn't really accept it himself.

3. He doesn't understand Agni at all. He especially doesn't understand the whole "friend" thing, and doesn't really get why he... likes it.

4. He is mostly motivated by boredom. Yes, he's starving, but only by choice.

5. He's much more intrigued by humans than he probably should be, but after all.. boredom. So he says.