Now I'm going to disagree with a lot of the foregoing - though I haven't seen the first episode of the second series! Sam showed quite a lot of empathy throughout the first series - for example, in his dealings with Leonard in the 2nd episode, with Joni in the fourth, with Ryan in the 5th, with Billy Kemble's sister in the 7th episode. Maybe he reins in his feelings so tightly because he's scared of what what would happen if he lets go?

(And from the spoilers I've been reading, it sounds a bit as if that is what happens here!)