So, basically, Summer thought Tom was ugly and too weak for her...
And he probably made around $30,000 a year or so at his *beep* card writing job and he probably sucked in bed also, not to mention he was a stage 5 clinger. I can't think of any other reason two people that have so much in common wouldn't work out in the end.
On a side note, I love the part in the end when Tom says that he "quit the office" and Summer says "that's great!" or whatever. In my mind I was thinking "So now you're unemployed? What's so great about that?"