Optimal level of arousal is the correct answer.
Each person has a different level of arousal and this level is perfect for them. In order to maintain this level, people will seek for activities that promote their own stimulation. In this case, Joel got his arousal elevated when he went out partying/clubbing, while Natalie was happier when she was able to stay home and read her mystery novels. Thus, their relationship was unlikely to succeed since their optimal arousal level was out of sync.
