Details: Seth always had a crush on Summer but she just thought he was a geek and didn't think anything about a relationship. When Ryan came to Newport, Marissa started dating Ryan and Summer started to develop a crush on Seth but he's dating Anna Stern and Summer becomes Jealous. After a while Seth decides he really loves Summer and decides to end his relationship with Anna to pursue a relationship with Summer and they both lose their virginity to each other. Summer and Seth broke up quite a few times for numerous reasons including Seth running away and Summer moving on and starting a relationship with Zach, Summer thinking Seth was seeing Anna again when really she was just trying to help him get into college to be near Summer, in the end they get back together and end up married in the flash-forward episode and last in show The End's Not Near, It's Here.
Details: When Seth runs away, Summer moves on and starts a relationship with Zach. Zach joined Seth's fledgling comic book club, becoming close friends with Seth. However, their friendship was tested when Seth learned that Zach was dating his ex-girlfriend, Summer Roberts. Zach and Seth decided to create a comic book together, called Atomic County, which was centered on Orange County teenagers who had super powers and related to the core four. During the creation of the comic, Summer realized that she still had feelings for Seth, and went back to him.
Hurt, Zach made up a story that he had met an Italian girl while in Italy for his sister's wedding; Seth and Summer quickly learned that the story was false. Zach and Seth physically fought over Summer at their comic book launch party, much to the embarrassment of both Summer and book publisher Reed Carlson. After Summer told them that she would not date whomever was involved with the comic, Zach left Newport Beach to work with George Lucas on the comic, and Seth became involved with Summer again.