There you are, rising from your seat as the house lights come up, dumbstruck with jealousy and outrage at the film you just finished watching. "How," you ask in shock and incomprehension, "did THAT film get into Sundance when mine did not?"
Yes, I've been in that seat too. I recommend reading an article by a two time Sundance alumni, who, like you, was rejected this year by the most dreamed-of, hoped-for, and disappointed-by festival in the country...
A Filmmaker's Life: I Just Got Rejected From Sundance