Now, I know there are tons and tons of AMG sites with indetail information, so this is going to just be a quick summary for those people who this is the first site they found, which is really unlikly, but anywhoo, this is just a small summary about the couple.
*Warning: Contains spoilers*
Was it destiny? The triumph of true love? Or just a wrong number? When a lonely, hapless (and girlfriendless) college students named Keiichi Morisato called from his shabby dorm to order some take-out food and got the Goddess Help Line instead, his life suddenly took a very strange turn. A lovely young goddess named Belldandy then appeared to him and offered to grant him a single wish, he wished for her, she granted it, and they stay together forever. Her own feelings don't seem to be considered, making her a slave of sorts. However, the situation is softened by Keiichi's terminal shyness: even though they're living together and he is not unaware of the possibilities, they don't sleep together. Also, she falls in love with him for real. Despite later opportunities to break the contract, she prefers to stay with him.
Later on in the episodes, it's revealed that Belldandy and Keiichi had met before as children and already had a relationship as friends. Although he was made to forget, she had genuine feelings for him. This brings up many possible reasons for how they met again. It may have been that Keiichi's connection to the Goddess Help Line was something Belldandy deliberately engineered in order to see him again, bypassing the usual procedures for selecting a worthy candidate. She may have been half-expecting for him to wish for her to stay with him. Although in the first few episodes it doesn't seem like this, but when Keiichi was able to see Belldandy's light, something he shouldn't have been able to see as a human, it may be possible that this pairing was done by an even higher level than Belldandy. Well, just a thought.
This information is from Animerica Vol. 10, No. 1.