With NPCs you have to build up the relationship which is a hurdle that must be gotten past. Then you get a bouquet. Then you have to build the relationship again. Then you can get the pendant.
With another player, you skip all of that. You could get married day 3 if you could get the bouquet and pendant, and then you'll get a stardrop LONG before you're supposed to be able to get one. Marrying another player needed to have a different process that prevented you from doing something like this.