The Lonely Orphan
I love Mamoru angst. I have this sick fascination of seeing my favourite character in pain and it doesn't have to be mental or emotional. Physical pain is good too; the more the merrier. Bishoujo Senshi Sailor Moon, being a shoujo series, doesn't have much Mamoru angst so it's all up to the fans to speculate and explore "what-if" situations.
The major theme that Mamoru is associated with is loneliness and solitude. Being raised in the orphanage, he must have been lonely. He has neither friends nor family. In fact, nobody likes him. Up until he met Usagi, he has led a tragic life. This situation is so stereotypical it isn't even funny.
I can understand that he doesn't have much friends because I can see that being in an orphanage kind've ups your cynicism and stunts your sociability skills. But no friends and no life whatsoever? I can't see that. He's an introvert, someone distant, a person who doesn't talk about himself and one who has trouble expressing his feelings but the guy should have had some interaction with another human being in his lifetime.
Having had no one to lean on for his all of his life, he's become incredibly independant and very mature for his age. And the environment in which he grew up acts an additional layer to the established societal mindset that a man is not allowed to express his feelings because to be sensitive is to be weak. So he adapts an aloof persona. Even so, it doesn't muffle his natural charisma.
When he's first introduced, he's wearing a tuxedo and insulting Usagi. He's confident*. He plays soccer so he must be a team player. We see him hang out with Asanuma which shows that he hangs out with his peers. He reached out to Asanuma when he needed reassurance; that proves he's initiative and simply kind. Chibi Usa quickly attaches herself to him; he's looked up to in school; Ami was curious about him; Usagi finds herself thinking about him despite his teasing; Mamoru's captivating. A person with all these wonderful traits, it's hard to believe that he doesn't have any friends. Maybe they aren't close to him, maybe he doesn't have much, but he definately wasn't an outcast who everybody hated.
Of course, that doesn't mean he wasn't lonely. To be in a room full of people and not feel like you belong or connect with anyone there, that's solitude. I'm just saying that he isn't the "type" of loner where he doesn't talk to or interact with anybody; that's unrealistic though he is still very much underdeveloped in the people relations' department. There are situations that he doesn't know how to handle. The "fight" that he had with Usagi because of the appearance of Haruka and Michiru being a good example. It is a true that Usagi was jealous as well, but she confronts him while Mamoru coped by ignoring it even though it clearly bothered him. Having no one to share his feelings with, he's somewhat emotionally instable. However, if pressure is applied to the right place, he'll break.
*As the series progressed, however, we see a side of him that isn't too sure about himself. He doesn't have faith in his ability to protect Usagi. It was like a familiar self-discontentment reawakend after years of suppression. After being with Usagi, he was forced to revisit his insecurities. All of a sudden, the questions about whether or not he was worthy, came back to haunt him. Though this time, locking himself away, putting on a cocky front and pretending that everything was okay weren't going to cut it. He wants to be the best that he can be; he wants to please someone; he wants her to be proud of him.