When Naoko wrote the story of Sailor Moon, she made Mamoru so loving, so gentle and kind, so noble, and she never had him break up with Usagi. However, the animators/writers had a different scenario in mind. They wanted heart ache, turmoil, they wanted Usagi to cry and they wanted to make Mamoru/Darien look like a jerk, so they went and ahead and had him break up with her. And what was their reasoning? "I know! We'll send him nightmares from his future self, who will test his love for Usagi. And the test will be seeing Usagi die if he stays with her."
Now poor Mamoru/Darien. All he wanted was to live peacefully and happily with Usagi. And yet, when he gets these dreams, all he can think about is the pain on Usagi's face. He can't BEAR the thought of any harm coming to her, and he especially can't stand the thought that IF he stays with her, she will lose her life. So what does he do? He sacrifices HIS happiness and breaks up with her.
Mamoru throughout the break up shows how he is suffering and how hard it is for him to break up with the love of his life. Yet he does it because his love is UNCONDITIONAL. He wants Usagi to be happy, to be alive, to live and breathe, and if by staying away from her, Mamoru can ensure that, then he will close his eyes, ignore the pain in his heart, and do it. That is unconditional love - loving someone so much that you give up your own happiness with them, so that they may be happy and live.
What would YOU do if you were in a similar situation? You've been given a death prophecy, telling you that the one you love will DIE if you stay with them. Would you be SELFISH and stay with the person, thereby causing the one you love to die? Or would you love the person so much that you would stay away from them? Life or Death? Your own happiness with the one you love or Death? Choose. You will find that you would most likely do the right thing, which is what Mamoru did.