Ah, that's one of the core questions of John Proctor's character in this play. Unfortunately, like several of the key elements of this play, we don't get to directly observe the action, and we can't see into his mind. All we have to go on is what he says, and what Abigail says, and perhaps the general atmosphere and general human psychology. Abigail talks about being a "wild thing." Perhaps he was drawn to her wildness. She also accuses his wife of being cold; perhaps John was frustrated in bed. Perhaps it was the boredom of a small community. Perhaps it was the fact that he should have power over her, and there is a long history of men who have power using it for sex. Perhaps it is that she wanted him so badly that he was tempted.