Normally, the first instinct we have when we feel an unpleasant mood coming on is to shut everything out. However, a study out of Tel Aviv University's School of Psychological Sciences suggests that if you come across something that has a negative effect on your mood, repeated exposure to it can actually lessen the effect it has on you:
"A bad mood is known to slow cognition," said Moshe Shay Ben-Haim, Ph.D.

"We show that, counterintuitively, you can avoid getting into a bad mood in the first place by dwelling on a negative event.

"If you look at the newspaper before you go to work and see a headline about a bombing or tragedy of some kind, it's better to read the article all the way through and repeatedly expose yourself to the negative information. You will be freer to go on with your day in a better mood and without any negative effects."
Of course, how effective this method is also depends on what exactly the bad news is, so too much exposure to a personal bad event can also have a more damaging effect if you dwell on it to long.

[Psych Central]