We all suffer and grieve in our own way. There is no correct way, and sometimes the grief we see in others is so strong that we feel overpowered.
That is Lee, his grief drives his life and it drives others away. They can't understand him, or the person he has chosen to become. He's allowed his grief and suffering to take his life over in a way that is hard to digest.
Powerfully acted and simply shot, Manchester by the Sea hurts to watch, but that's grief sometimes.