Gerard had said in an interview (not in a video, but in a written one) that he was getting back to alcohol again (back during the Revenge era he became an alcoholic, remember?), and that he had to let something die, either the band or him. Of course, he chose to end the band instead.
Another reason to which a lot of fans have theorised is that Mikey didn't want to be in the band anymore. And as Gerard said: "My Chemical Romance isn't My Chemical Romance without Mikey." In fact, without Mikey, they wouldn't even have a name for the band. (Gerard has also said that if they were ever to return, Mikey probably wouldn't be back and they would have to change the name.)
My Chem is broke up not for a specific reason but they said, "Being in this band for the past 12 years has been a true blessing. We've gotten to go places we never knew we would. We've been able to see and experience things we never imagined possible. We've shared the stage with people we admire, people we look up to, and best of all, our friends. And now, like all great things, it has come time for it to end. Thanks for all of your support, and for being part of the adventure.
—My Chemical Romance" In an interview Gee said that it wasn't anybody's fault it just felt like it had gone on for a while so yeah they had great times only they wanted to try other things I guess. This is the best I can do for you so I hope I answered yours & many others' question.