The main antagonist could be a character within their own group who has turned bad. This person might be jealous of the relationship between Steve, Tony and Peter. They spread rumors and lies, which ultimately leads to Steve believing that he is not welcome or that he is causing more harm than good, and so he leaves.
Maybe it's an old enemy of Steve's who wants to isolate him. By driving a wedge between Steve and his friends Tony and Peter, the antagonist can make Steve more vulnerable. This enemy could use false information to make Tony and Peter distrust Steve, which in turn makes Steve think it's best for him to leave.
It could be a new villain who has the ability to manipulate minds. This villain could make Steve believe that he is a danger to Tony and Peter, causing him to leave. For instance, the villain might implant false memories or thoughts in Steve's head, making him think that his actions might accidentally harm them in the future.