It seems with all the evil acts he has done in his entire adult life.
From murdering and restoring Bor only to have him killed.
Or having Asgard be brought to it knees by Osborn and his cronies.
Or during Defalco run where Loki killed or was it seriously injured a friend of Thor and numerous other examples.
Consider it from the point that Loki never really had a chance. He lived for so long in Thor's shadow and all he really wanted was to be seen as an equal to Thor in Odin's eyes. I think the way Loki was portrayed in the movie illustrates this the best, especially the scene in the vault with Odin and Loki "discussing" what Loki was.