Upon meeting and inevitably joining the Dragon Riders, you and your dragon had joined the group in dragon training lessons. As Hiccup explained, today's lesson was on bonding with your companions' dragon as to train with a dragon that isn't your own. You had originally been paired with Hookfang, but as Snotlout decided to open his stupid mouth and said he wanted to swap as Toothless was a "cripple", his father was told of this and he sent home with Hookfang in tow, much to the other Riders' relief due to being disgusted with Snotlout's attitude.
However, with Hookfang gone, you were partnered with Toothless instead, which you were rather happy about. In your words, "Snotlout doesn't know what he's missing". After a rather cheered up Hiccup taught you to work Toothless' fin, he went to his dragon for the day and everyone spent the day bonding with their assigned dragon... Or HALF of a assigned dragon, in Barf and Belch's case.