Shield has worked his whole life to become a prominent member of the guard in Equestria to prove himself among his peers. Through hard work and sheer dedication he was recognized by Shining Armor as a guard of extreme dedication. He was promoted to personal bodyguard of royalty, and just as he was celebrating he realized the one he was to protect was none other than Princess Cadance. Wife to Shining Armor, mother to their child Flurry Heart, and the first and only pony that Shield's ever fallen in love with.
(This story takes place in an alternate timeline after Flurry Heart is born, assuming all previous events had taken place prior to her birth)