Mamoru's purpose, center, and happiness hinges on his Usako. When they first encounter each other, they both get first impressions of one another that only scratch the surface of their personalities. They tease each time they meet, yet at the same time a mutual attraction develops.
At first Usagi thinks that Mamoru is just some cocky guy, too full of himself to be nice. As time goes on, they find themselves staring at the other when they aren't looking. Mamoru is the first to pick up that Sailormoon is Usagi. After a battle in which she faints, he carries her back to his apartment and tends to her.
It is here she finds out Tuxedo Kamen's true identity, and she also finds out just how gentle and loving he can be. As time goes by and they conquer many battles together, they become incredibly close. Their dreams, hopes and aspirations become as one - to protect their beloved planet together.
I'm not sure how well Mamoru would do without Usagi by his side. He is intensely insecure and has a cocky shell which may repel other females, despite his good looks. Usagi, slowly but surely, helps Mamoru to become more confident in himself as a person and as a warrior. Even though he constantly doubts his abilities, Usagi always truly and honestly believes in him. She never sugar-coats her opinions or feelings towards him. Usagi is the bubbly familial warmth that Mamoru needs to balance him, and Mamoru's calm intellect balances her.
They truly fit like two pieces of a puzzle. Their love is challenged continually, but they are always ready to face the odds. This unique, surviving bond will be crucial when the rise of Crystal Tokyo occurs.