It's not just his happiness he's giving away, he's giving away the life he would have had with MJ. He pretty much guilt-tripped MJ into going along with the deal. So MJ's suffering too at Peter's expense. Heck, because of Peter's deal, the child they would have had in the future no longer exists.
This is not even mentioning that Aunt May told Peter that she was tired of living and wanted Peter to raise a family of MJ. (She even told him that even if she hadn't been shot, she didn't have much years left anyway) This is not mentioning the fact that God told Peter to have faith. This is not mentioning in that making a deal with the Devil, Peter willingly helped the Devil achieve a victory over God.