Marius rescued Armand from poverty and then later from death by giving him the dark gift. They were together too short a time - only a few years, which isn't long as far as vampire relationships can go.
This is probably the most romantic relationship in the Chronicles and it ended quite tragically even though neither of them died. Armand was torn away from Marius by Santino after watching Marius being burnt alive by Santino's gang of religious zealots.
"All comfort, all goodness, all hope was burning in this black figure which my eyes would not let go, even as it dwindled, and lost all perceptible form.
Marius! My will died.
Armand didn’t really move on completely at first because he believed his maker, and the only one he’d loved, was dead. Their love is still there as we can see in Queen of the Damned and in The Vampire Armand when Marius made Armand two fledglings. Their reunion in Queen of the Damned could bring you to tears.
“I love you,” Marius whispered suddenly, passionately as a mortal man might. “I have always loved you. I wish that I could believe in anything other than love at this moment; but I can’t.”