Tom meets Summer, and think she is the one. Summer doesn't think the one exists, but Tom hopes to prove her otherwise. Instead of listening to her warnings that she isn't seeking anything Tom continues to hope that will change. This movie lets you know upfront how everything will turn out, but somehow it still manages to keep you intrigued, and even spark the hope that they will work out.
In exchange both of them learn from each other what it is they are missing from their lives. A lot of people judge the movie as being depressing, and only being about a breakup. It's about so much more than that though. It is more so a life lesson. I can see some of myself in Summer, and Tom, and I've experienced both sides of the scenario that plays out in the movie. Everyone is so harsh on Summer as if they have never had to break someone's heart. Even if you have never broken up with someone I think we've all caused someone to feel rejection.
I have many favorite movies, but this one seems to consistently match my mood. I can watch it when I'm in a good or bad mood. I have never found it depressing, but more so encouraging.