Sans was fine with what he had. Harvesting sinful souls had been what he'd done for the past few centuries, and he didn't enjoy it, but he could handle it. Unfortunately, after accidentally killing the one he loved, he went insane and suicidal, with parts of his soul missing. Toriel helped him, regaining his sanity. However, his soul is not yet complete and he is still partially insane. It's back to normal, for the most part. However, he can still become uncontrollable and temporarily insane 
