Here's another reason, and a Rules one: She talks to him first. And he slams right into her, not hearing her, because unbeknownst to her, he's just gotten fired. Maybe that wouldn't have been so very bad, except that when they meet up accidentally again and again, she talks to him first to him again and again, requesting, during one party scene (actually raising her hand!) that he play "I ran." Yeah, he did run, Emma Stone character. You should have said, "Next!" and moved on. But no. She keeps talking to him every time they run into each other and when he's on tour she calls and says in the neediest tones imaginable that she misses him. Then, surprise! He's in their tiny grungy apartment making dinner for her. But not so surprising, they argue. The moral of this story may be that she got bored with him because she chased him. He didn't chase her. Not in the beginning. Into which the end is rolled.
This kind of originality--not to mention philosophy--is another thing missing from a film styling itself as paying a tribute to Singin' in the Rain: