This ship grew on me. 

I imagine that these two like to talk over come good cider, swapping stories of saving Equestria and catching Flash up on social norms and pop culture. (No, Magnus, a record player does not have little ponies in it that play the music)

Flash Magnus integrated pretty easily into modern Equestria, but the poor guy just couldn’t up his dating game. A royal guard should be charming, after all. Unfortunately, when you’ve been in limbo for a thousand years, you don’t have a lot to talk about with the ladies.

Luckily, Rainbow was interested in what he could talk about and was more than willing to suggest her favorite music, books and movies for him to consume. The two of them talked and argued about their tastes in culture for hours at a time when they could. 

They also developed a natural rivalry (Flash being the fastest pony of his era and Rainbow the fastest of hers) and spent a lot of time racing and training together.