Because he is the older brother, he is responsible for the safety and care of Lumine.
What will he do now knowing he and his best friend like the same person? He wanted to submerge himself in Liyue and learn more about the foreign place on his own, and fortunately, he got more chances to speak with a certain someone on his journey.
But because of a simple mistake, and a simple flaw in his plan.
It was after sometime when he realised he was the one falling into the hole of feelings, when he realised that 'certain someone' was in fact, more than an acquaintance, more than a really good friend.