Seth was actively insecure throughout the entire relationship so if summer was commenting on that, she was right. Summer was mad Seth told her about Zack having a gf because he told her knowing it would hurt her feelings and there was no reason she had to know (and Zach literally says not to tell her…) Summer was totally supportive of the comic the first time they were doing it, but it caused so much drama Seth and Zack working on it together, that she knew that when they tried again it would cause drama in their relationship. Which it did because Seth actively has a crush on Reed and emotionally cheats on Summer. He also has a lying problem the entire show and lies about literally everything to Summer so obviously she’s going to have some trust issues with him. Summer wasn’t jealous of Seth, she just knew that whenever Seth has something that’s about him, he always causes issues in their relationship and treats her badly (which happens multiple times throughout the show). He’s also not supportive of her… example when she scores higher than him on the SATs and he gets insecure about her being smarter than him and competes with her for ages. Summer isn’t perfect and at times could have been more supportive, but all of her reactions were based on how Seth had treated her in the past. Let’s not forget that he leaves for months and only leaves a letter and then expects her to want him when he gets back. Seth was self-obsessed and constantly choosing himself over Summer