Narcissa Malfoy is asking Draco to find a woman to be the new lady of the manor. Draco doesn't want a wife, for goodness' sakes, he's already dealing with so much at the ministry. And now, he's being told to get married to some woman? Yeah, no way. So what would any cliche story make him do? Ask you to pretend to be his fiancee. Why? Well. Why not? It'd certainly lift the weight from his shoulders. So when he asks you, you think he's an idiot. Why would Narcissa Black Malfoy ever believe that Draco's best friend would get married to him? But he's already invented a whole backstory. And you know what? It's irritatingly plausible. So what do you do? You say yes. Of course you do. How else would the story progress? Not to mention, Narcissa is staying in the manor for... Oh, I don't know.. One whole year? So you have to do this whole thing for one year just to save Draco. Why couldn't be just use that time to find a girl he actually likes? Who knows at this point.