I had the same sentiments, still have, as you. The way he was forced to become the princess's summoner totally rubbed me the wrong way, since before this he was okay with living a simple life. However, throughout his time spent in the capital he has seen the injustice of how commoners, especially commoner summoners are treated. He also saw how the princess was fighting this injustice, giving the commoner summoners a chance with better support and education. Lastly, how the commoners fought tooth and nail to justify their existence during the duel, he saw that he needed to use his powers, even if he still questions the motives of the royal family with him. At least he is helping the commoners.