She marries because she loves the houses and the leisure and the money, money that can buy anything in the world, floor-length dresses and bright pink shoes and makeup that actually suits her skin colour. All the things the wizarding world doesn't have and doesn't care about, and all the ones it does too. Money buys fame and beauty and goodwill, and she will never lack for any of those again. (Nor good food or clean clothes or time, the one thing she values above all else, but she will never think this to herself too loudly. That might be asking for trouble.)
She can't stay married because she hates the names and the looks and the expectations, hates the slurs they voice without pause for thought. She can't stand to be locked in, whether it be to a house or a family or anything else at all, so she laughs when they die and leave her to be glad and happy with her self.
But when the mourning is over, the looks start to sour and the whispers start and the money runs out and she's never been good at holding down a job, so she finds someone she can tolerate for a few years and does it all again.
By the announcement of her fourth wedding, her son looks at her as though he's asking "Do you want to get caught?!", and she laughs at him and shakes her head. No, she doesn't. But she is who she is, and even if she hasn't found happiness yet, this is the only way she knows how to look.