We played horribly, simply as that.
Atlanta won because they played with heart when it mattered most - while Boston spent way to much time in way too many games standing around and watching on defence, and allowing wide open threes and layups.
When you play in the regular season against scrub teams, maybe you can play that way and still win games. You cant get away with that in the playoffs, especially when you have key players out.
If the Celtics lost, but did so laying everything on the line, then I'd be proud of that. But they lost thanks to incredibly inconsistent levels of effort and execution. They really did dig their own grave, as the saying goes.