Im a HUGE Loki fan!
I love answering questions like these.
The answer is that Loki fought Thor out of Jealous and Hatred. Loki also was casted aside by his own father adopted father ( Odin ) and was loved more by his mother. Loki now sees in The First Thor Movie that his was originally a child of Laufey. He has also learned that he was an monster, which made his whole life just a big lie and uncovered the truth why Odin loved Thor so much than Loki.