When we last saw Ally she was in shock over her encounter with Billy. Ally realizes her life is in crisis and that everything she does from this moment on could mean a lifetime of happiness or bitter disappointment. That's when she makes a decision not to be passive and do something about her life.
The "Love is wasted on you, Aly!" scene might be the best of the whole series for me thus far. Like wow!!! I don't ship Billy and Ally like at all, but they're such a fascinating failship. I love the tragedy that is their relationship.