What were Summer's motives at the end?

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Summer wanted to friend zone Tom so she could feel better about herself. She didn't really care about Tom, not even on a friend level.

If Summer actually was a real friend. She would have broke it off right after the bar fight. (Instead of having make up sex)

I'm not saying Tom was perfect either. It's obvious Tom was blind to many facts. Such as Summer ignoring a lot of the things he like to do, or failing to meet certain expectation which aren't reality.

Regardless, of Tom's faults, Summer was in control of the relationship. She refused to take a stance on how she felt. Summer was leading him on, which I'm fair sure she recognized this, after her marriage and realization that love actually existed.

The fact that Tom was right, made Summer feel guilty. She used him and that is the main reason why Summer decided to visit him. She finally understood what Tom had been getting at about love and she felt bad for leading him on. She admits this to Tom and tell him he was right, but that unfortunately she wasn't the one for him.

Maybe Summer does care a bit for Tom, but a real friend would have spoken up a lot sooner and not dragged things out. True friends are those who dare to let you know when you are mistaken, rather then agree with you over the wrong thing.

I think she wanted him to know that she cared about him and was sorry that he found out about her engagement that way. That, and that he was right all along about love. It has to be right for both people though. After that, he was free of what might have been. When he said he really cared that she was happy, he did the same for her, freeing her from guilt.

When they meet at the park bench at the end, Summer mentions that she now loves that spot ever since Tom took her there. I got the impression she didn't seek him out, it was just a same place at the same time scenario and she took the opportunity to try to smooth things over.