The Black house, was always Lonely, your dad didnt even remeber about you anymore, he chose the "golden boy", "the boy who lived", and here you are, hidden behind his shadow. You've had enough.
Youre parents werent a fan of you, they preferred only one kid, and you were Harry's twin, Harry still got the scar from voldemort but you did too. And they lived, its just Christmas with the weasleys, now