Sorry Frank Sinatra, but love and marriage don't go together like a horse and carriage in Mansfield Park. Love is sort of like cheese to marriage's hamburger – great to have, but not absolutely necessary. Marriage often teams up with things like money, pride, revenge, rebellion, or faulty assumptions. Love is more or less optional when making a union. And marriage is basically a crapshoot – everyone getting married takes a huge risk and gambles on it being happy or not. Even love cannot ensure that a marriage will be a good one. Ultimately, though love and marriage don't necessarily go together, marriage and social status do. For women in particular, marriage is an opportunity to rise up the social ladder. Lady Bertram, for example, becomes a wealthy woman through marriage, while her sister Mrs. Price ends up in poverty after marrying a sailor. As a result, when considering marriage, love often takes a backseat to more practical concerns.
In the end, marriage seems to be a reward, or at least a consolation prize, for the best-behaved characters, with the most moral folks getting the best marriages and the least moral not getting married at all.
Fanny is the only character in the whole book to marry just for love; every other character who marries, or who tries to marry, does so for both love and for other reasons.