In The Reader by Bernhard Schlink, why did Hannah peek on Michael while he was getting undressed for his shower?

This may seem like a dumb or strange question to ask, but it really intrigues me. Did Hannah peek on Michael so that she could get revenge/get even with him after he peeked on her getting dressed earlier in the film? Perhaps to let him know how she felt when he peeked at her? She did seem a bit upset and irritated by his presence when he returned to see her after he stormed off when she had caught him peeking at her. So perhaps she took advantage of the situation to give him a taste of his own medicine. Or was there another reason as to why she peeked on him, especially because she promised she wouldn’t look while he got undressed for his shower.

Also, for those of you that have seen the movie, how similar are the two events? Is there anything different between the two? I’m just wondering since I like to see the comparison between a written story and actually adapting it into a film.

Once again, sorry if this seems like a strange question to ask.