My wife has never forgiven me for failing to come up with a romantic way to propose to her. Still, there's one guy who I feel confident I'm more romantic than, and that's Tyler Cowen:
Presumably a proposal "out of the blue" is more likely to be accepted, and offered, when there is potential disagreement about the correct "market prices" of various potential partners. A lower status man might try to snare a higher-quality woman, perhaps by catching her off-guard, or he might be trying to signal, with his daring and panache, that he is higher status after all.
The most likely ones to accept such proposals are women who are unsure of their "quality," either on the mating market or in unmarried life. Accepting the proposal takes on one kind of risk but relieves the woman of another.
I happen to think the following is the best marriage proposal ever, but I await Cowen's economic analysis: