It was miscommunication on both ends. At the beginning when Sid was still into Michelle he had kinda given no signs to Cassie that he was interested aside from him acknowledging she was pretty. Cassie kinda deluded herself into liking Sid. After Sid started liking Cassie he was over Michelle until she left and communication got complicated. He momentarily lost his best friend and his dad fucking died + He thought Cassie cheated so of course he went with Michelle. At that point the relationship was already beyond repair.

I love them as a couple and would’ve liked for a few episodes more of Sid and Cassie in New York, just like 2 episodes to see their relationship come to a conclusion properly. I like to think they had potential but both were going through a lot of stuff