Re: Leave your problems at the door, they'll be waiting for you when class is over.
I've never been told that, but I noticed that I automatically did exactly that when I just started. I went through a rough time in my life, but during training I was concentrated only on "how the hell do they make this look so easy?" and attempting correct ukemi.
Now that the rough times are over and I'm becoming more proficient, I tend to be less concentrated somehow, but I also smile more.