This was actually the only canon pony ship I liked for a looooooong time when Horse Women was newer and Next Gens seemed dumb to me at the time lmaO.  
I just really liked the hypothetical dynamic they’d have as a couple. uwu
 
I was always fondly attached to this piece for inspiration: fav.me/d4dmf45
 
-For the sake of their vast age gap I headcanon the two did not see each other personally over the span of Pip’s childhood and into adulthood where, he chose to serve in the Royal Guard’s navy  
-When they do happen to reunite most likely at a harbor while Luna is attending a sendoff of a ship. Luna tends to present herself as being standoffish and normally lacking in conversation; Pip intended to amend that. It is entirely on Pip to manage a single word of response out of the princess, having asked a multitude of colloquial questions including whether or not she recollected who he was.  
-In order to get the young stallion to desist she finally blurts out “Yes!” before storming off in utter annoyance. Awed by the response it didn’t take long for him to ensue after her.  
-It took a while for Luna to warm up to Pip who continuously now had managed to find her despite her countless attempts to lose him in dozens of attended events. Eventually she gave in and humored his endless questions about her life, duties, hobbies, interests, etc.  
-Pip had become quite smitten early on in their interactions, but Luna restrained herself to only friendship with the stallion out of guilt, knowing full well how time would play out such a relationship in the end.  
-Regardless of that fact, which Luna informs him of, Pip is the one to encourage they become a more serious couple. He even agrees to only seeing her privately and never displaying affection toward one another in public. Though every once in a while he manages to plant a kiss on her out in the open and she gets very flustered.  
-To the general population, very few knew of the affair between her and Pip. Luna would have liked to think she kept her relationship a secret from her sister, but Celestia was very well aware and enthusiastically supported her younger sister finally reaching out to feelings she had long since buried away.  
-Unlike her older sister who’s more prone to sleeping around than settling down with one individual, Luna had very little relationship experience prior to Pip  
-In the episode A Royal Problem it becomes apparent that Luna isn’t prone to smiling, which I’d like to believe is something Pip constantly encourages her to do. It’s very rare that she does so in public; even more so to get her to laugh, which is something Pip has managed to do on more than one occasion during their private meetings together in the dead of night  
-Pip cherishes Luna’s laughter and sees it as a sign of her loosening up ever so slowly