Its a typical junior mistake to marry for love/lust and not think a bit on top of that, in this case I blame parents who don't have some hard talks with their kids explaining them not-so-rosy parts of adult existence. Like initial enormous physical attraction wanes over time, kids crush most of remaining, and what still remains are 2 people and how they treat relationship and each other with that lust tuned down eventually to 0, under various, often not so nice situations. But our parent's generation didn't figure it all out, in contrary the amount of actually nice relationships in higher ages ain't that high.
I didn't have such prep talk neither, nor do I know anybody who had, and had to figure it all on my own via rough trials and failures till finally figuring myself and women out, and then happy marriage (so far, hard knock on the wood). Its like expecting everybody to be sophisticated engineer, learning them to count on fingers and throwing them out and good luck, I am sure you'll figure it out eventually. Some do, some don't. Most don't I'd say.