Mamoru is rather a unique character in the AKB0048 anime. He’s unique in a sense that he started out as that popular guy in class and he eventually learns to love idols. He’s pretty interesting already in that regard but rather I want to focus on his relationship with Yuuka.
At first Mamoru was utterly against Yuuka’s decision of joining AKB0048. Few episodes past, he’s now part of the wota and is an idol fan. In episode 12, Mamoru mentioned that he loves the Yuuka who is now an idol, but also mentioned that he no longer loves the Yuuka outside of the idol context and tells Yuuka that his relationship with her is that of a fan. That basically is Mamoru’s way of calling it quits with Yuuka.
My take on this is that Mamoru is aware that if Yuuka becomes an idol, she won’t have any time for him. While the anime paints Mamrou’s opposition to Yuuka’s desire to join 0048, I was able to highly relate with this. Yuuka does the inevitable and joins 0048 and sometime later Mamoru becomes a wota. Fast-forward to events in episode 12 when Mamoru nicely calls it quits with Yuuka, I think that at this point Mamoru has realized that Yuuka is too far away from his reach and that the only thing he’ll ever be is a fan to Yuuka and with that realization, he calls it quits with Yuuka. I mean who wants a girlfriend that he can’t spend time with physically? In my opinion, Mamoru is just being a practical guy.