Sunburst got his cutie mark for magic, as we all know. His parents were very impressed, and as a descendant of Star Swirl (hinted at in how he looks in the trailer), his parents forced him to study more and possibly become the next magical council for the princess. This caused him to become very depressed (as he didn't want to use his talent for this) and it drove him and Starlight further and further apart. Before he had to leave, he told Starlight about how much he hated the fact that he got a cutie mark, prompting her new theory. Now he seeks revenge on thoe who caused him so much pain.