Simply put, I expected that right-clicking a (tamed) horse when golden carrots were in off-hand would cause that horse to go into love mode. Instead, I mounted the horse. Similarly, I expected that when holding a saddle in my off-hand, and right-clicking on a tamed horse, that it would put the saddle on the horse and mount, but instead I just mounted the horse.

I guess that might not be unexpected, since my main-hand "right-click" action would be to mount the horse, and that takes priority, but I suppose I expected use of a usable item on the horse to take priority over the "default" option to mount the horse.