It's explain in a chapter during the second season. It was cause Shen didn't take care of his relationship with Ren and only focused on school. It Said something like he wasn't there during valentine's day or the birthday. He didn't even send a text to him so Ren probably dumped him cause he suffer his absence
thank you !
@Romy has explained it well, I just want to add that Ren's friend told Shen, "When were you when Ren caught the flu?" and "How many times have you seen him in two years?". So it seems Shen didn't see him much, because he was so busy with the school/finance thing. He didn't prioritize their relationship. And maybe something happened that made Ren decide to break up. We'll see in the future chapters.
Why did they break up?