I would've had someone tell him to move on from Rarity or for his more mature self (so somewhere in the later seasons) to come to terms with moving on himself. It breaks my heart that he didn't see that he was just being used for Rarity's personal gain after a while. In the beginning, she at least did favors in return but it later became a "Spike is my little servant" deal.
If Spike had broken free from this, he would have focused more on himself and his destiny.