What makes Jerome mad in Chapter Five?

From the text:

We snarled at one another in this strain for the next few minutes, when we were interrupted by a defiant snore from George.  It reminded us, for the first time since our being called, of his existence.  There he lay—the man who had wanted to know what time he should wake us—on his back, with his mouth wide open, and his knees stuck up.

I don’t know why it should be, I am sure; but the sight of another man asleep in bed when I am up, maddens me.  It seems to me so shocking to see the precious hours of a man’s life—the priceless moments that will never come back to him again—being wasted in mere brutish sleep.