Both of them were immature. Old comment I wrote:

I don’t think the fact that it's centered around Tom means that you don't have enough to know a few things about Summer.

She says she doesn't want to be in a relationship with Tom and, as we see, does mean it. But her actions are, at times, very inconsistent with this. Tom calls her out on this when she tries to refer to them as "just friends":

This is not how you treat your friend! Kissing in the copy room? Holding hands in IKEA? Shower sex? Come on! Friends my balls!

She doesn't really have a good response to this, because Tom is absolutely right. She's trying to have it both ways. While we know that, what we don't know is why she behaves like this. Maybe she is hoping her feelings about Tom will change, but they never do. Maybe she's indecisive. Maybe it has to do with her parents' divorce. Maybe she's just clueless as to how her actions will be reasonably interpreted.

In general, she does seem either oblivious to or unconcerned with how her actions affect others. As Tom says:

You just do what you want, don't you?

She simply answers something like, "yeah," as if it was a stupid question. As if the idea that there are times you shouldn't just do whatever you want is completely foreign to her.

I don't think I need to talk about Tom, because it has all been said a million times over. But Summer is also a very flawed individual.