He's a loser dork who looks very cool and suave until you actually get to know him and his loser ways, you mean.
No, but really. Pretty much Sebastian's only endearing trait is that he tries so hard. He's told by another character at one point to be more doting and parental when Ciel is sick, and he's pretty much "...kay, um. /tucks in? attempts to spoon-feed? :D?"
Spoiler alert: Ciel orders him never to do that anymore.