Love. Perhaps because of her own experiences, from the time you were little you can remember your mother telling you to marry for love. She spoke about love like it was The Thing To Which We All Aspire. Perhaps, to your mother, the consummate Slytherin, it had fulfilled an ambition of a sort. How can you tell her you aren't like her? How can you tell her you want to be a good person first, an Auror second, and so many other things before wife and mother?
You can't tell her, and what's more, you can barely even admit as much to yourself. After all, your mother has reminded you many times that no emotion trumps love, no achievement makes up for it and no prize is more to be sought. So you tell yourself you want to be in love.
You want to be in love so badly that other things don't matter, details like whether you are really in love. Once he accepts your love, you know, then you'll feel that thing you want to believe you've been searching for all your life.
It doesn't matter that he's tall and lanky. It doesn't matter that he's poor, or nearly old enough to be your father. It doesn't matter that he turns into a monster once a month. And it definitely doesn't matter that he's in love with your cousin.
You want to be in love. Everything else is in the way.