I'd like to highlight the beginning of this especially. Sam meets Sybil in Guards! Guards! and falls into a relationship with her after. In Men At Arms, when he's coming up on his wedding, we get the following passage:
Be honest with yourself, Sam: do you care for her? Don't say 'love,' because that's a tricky word for over-forties.
In the end, of course, he does marry her; he decides that he does care for her, and that his care for her is enough to offset the parts of his life that he's willing to compromise to make a life with her. That this is something he wants, that they both want, even if they aren't madly in love.
He falls more in love with her later, but I want to emphasize that even if he never had, that would have been all right. It was enough for them to get married because they cared for each other and made a good partnership. It was enough that Sybil wanted to be married, wanted a partner to share her life and her inheritence with, and wanted Sam Vimes to be that partner. It was enough that Sam liked her and cared for enough to be that for her.
It's fine to make a life together with someone you aren't romantically, or sexually, madly in love with. It's enough that you care for one another, and that you make a good team. It's more than enough.