Karma, Bitch! -bxb-
Robin hates his alpha, mostly because he was an inconsiderate jerk that he's known for 15 long overbearing years. Derek was basically a constant reminder for him, with his own words, "You're not worthy of having a mate!"
When Derek's 18th birthday came, the day he'd waited for all his life to finally gain the alpha position and find his luna, he knew that the moon goddess had a personal grudge on him. His mate, was none other than the town's little 15 year-old omega. To this, Robin smiled, "Congrats on getting a worthless mate!"
And a series of other stories.