Basic Trope: In a World where marriage for money and socio-political alliances is the norm, a character chooses to marry someone they know and love already and want to spend the rest of their lives with.
Exaggerated: Princess Alice elopes with Prince Charles, then tells her father what they did.
Downplayed: Princess Alice is given her choice of several approved men, and may pick the one she likes out of them; with luck there will be one she can love.
The marriage was arranged with no input from Princess Alice or consideration for her feelings. (Especially true if King Bob the Nth is particularly unlikeable.) Prince Charles better fits the qualities of the kind of man she'd like to marry (plus she knows him better) than King Bob.
In the society in which Alice actually lives, marrying for love and companionship is normal; in her family's cultural background, Arranged Marriage is the norm, and it's an issue of The Generation Gap. Alice's parents want her to continue cultural traditions, and Alice wants to do what she sees her peers doing.
Alice gets engaged to her longtime boyfriend Bob, but begins eyeing (and eventually leaves Bob for) Charles (despite him being average-looking and kind of a Jerkass) because he is rich and well-connected.
Alternatively, Alice's society considers dating and marrying someone of one's own choice the norm, but Alice opts to have an Arranged Marriage.
Princess Alice is perfectly OK with marrying King Bob the Nth, even though she doesn't know him.
Zig Zagged: Princess Alice has second thoughts about marrying King Bob the Nth, but then she meets him and finds she actually likes him and finds him attractive. Then she meets Prince Charles, and falls in love with him, but still goes through with the marriage to King Bob. Then she carries on an affair with Prince Charles and eventually marries him after King Bob dies.
Averted: Alice faces no restrictions (be they political, religious, cultural, class-based) on whom she marries (or doesn't marry.)
Because Alice and Bob want to Marry for Love, they annulate their marriage, making her children illegitimate. They are snubbed of their inheritance, ill-threated by the stefather their mother married, and those who don't enter into dangerous professions to earn enough to live make marriages of conveniences. When Alice reminds her daughter with great dismay, Caroline, in front of Caroline's newborn baby, that she hasn't married for love, Caroline snaps and proudly replies that she, at least, married a good man, and made a sensible choice for her and her family. And then she gives Alice a "The Reason You Suck" Speech.
Alice's parents butt out, on the grounds that if Alice marries someone she likes, there's less likely to be an affair, a divorce, or other scandal. (And if he just happens to be rich...)
Alternatively, if Arranged Marriage is not to be done away with In-Universe, her parents take steps (probably for the above reason) to make sure they get to know each other and that they actually like each other; a Child Marriage Veto is permissible.