why are Marguerite and Percy unhappy with each other in the beginning of the book

When we meet Percy and Margot, they seem to be unhappy, miserable, and sorry they'd ever gotten married in the first place, but there is far more to it than that. There's a specific scene in tne novel where we see that all is not as it seems, and it comes from the way another character views them from the outside looking in.

Lord Anthony see Percy look at his wife longingly (this isn't done), and when Margot begs her husband for help and bares her soul to him he turns his back on her...... but he really doesn't. There's nothing more he wants to do than everything she's asked, but he won't because he'd never allow himself to show it.

Thus, we have two people, still newlyweds, and still in love..... but we also have pride, hurt, and necessity, which provide enough angst to keep us rooting for them.