There’s nothing juicier than a character begging their love interest not to marry somebody else. In most cases, said character has buried their feelings, thinking they can’t be with the object of their affection for some reason or another. But their loved one’s impending nuptials (or a situation heading in that direction) force all their emotions to the surface, and they can’t hold it in any longer: They just can’t let their love interest spend forever with anyone but them. (Is it a selfish act on the confessor’s part? Sometimes, but we’ll still eat it up.)
Some “don’t marry him/her” moments are big and dramatic, occurring on the actual day of the wedding. Maybe it even comes mid-ceremony.