There are two different types of love in Equestria. There is love that can be acceptable. There is love that is strictly forbidden.
Shining Armor and Princess Cadance: Acceptable.
Flash Sentry and Princess Twilight Sparkle: Forbidden.
It all started when meeting one another for the first time, there has been a myriad of antagonism and rivalries between the parents of Twilight Sparkle and Flash Sentry. There is no friendship between the families at all, only anger and hatred. Nopony knows what has caused all of this abhorrence to them, though the animosity between the Sentry family and the Sparkle family—is breaking the hearts of Flash and Twilight.
To make matter worse for them—they are forbidden to be together.
It is even more difficult when the two are working with each other, with Flash Sentry as a Royal Guard and Twilight Sparkle being the Princess. Despite this, they were unable to be together because according to their parents' demands