Once you were child, alone, wandering around the forest you found an egg of a dragon. You hid it all this years and after sometime you had to go South with our father and sisters. The dragon collapsed from the egg and it was getting bigger. Queen Cersei Lanister told you that if you don't leave the dragon she's going to kill it. You didn't have an option so you let it go.

A long time after, queen cersei wanted to end the bloodline of Starks. He gave commands to kill them all.