Surface contrast - at first glance they couldn't be more different; one's an athletic tomboy and the other very much feminine dressmaker. Stark contrast like that can make for good ships; tomboy/girly girl combinations are effectively a staple of a lot of things for a reason. Core similarity - though seeming to have little in common, their personalities are actually closer than they are apart. Both are bold, assertive, strong willed mares who ultimately believe in doing right by others. They're also the ones with the most artistic energy. Rarity is an artist while Dash is a showmare. They're both the ones with dreams bigger than where they are now; they desire fame and attention when attaining it doesn't clash with their core values. Being more on the same page than a lot of the others I think adds credence to the ship. Appreciation for one another's art - Rarity expresses delight in watching Rainbow Dash fly and do tricks (she uses the word "dazzling" for instance at the start of Read it and Weep). She also evidently talks Wonderbolts with Dash to the point where she can recall specific details (Sweet and Elite). When she herself got the ability to fly (Sonic Rainboom), she found it absolutely elating. It can be concluded that Rarity really thinks flying is cool. What Dash thinks of dresses is a little harder to figure out. She is shown to protest at the idea at times (though she hardly has AJ's disdain for fancy clothing), but there has to be a reason she was there modeling for Rarity in Swarm of the Century in the first place. In Suited for Success, she's the only one who doesn't give Rarity a series of specific suggestions; this is frustrating for Rarity, but I think it speaks to a certain level of trust she has in Rarity's ability to make something she'll appreciate. The events of Sonic Rainboom - This is one of the more important episodes for both Rarity and Dash. Rarity encourages the others to follow Dash so they can give her support, but ends up wrapped up in herself to a shameful extent wherein she ends up causing Dash pain and ultimately ends up almost falling to her death. Despite how she's acted, Rainbow pushes herself to do something she hasn't been able to do previously with reliable frequency so as she can save Rarity's life. Rarity is clearly emotional and reflective at the end when she asks for forgiveness and when she makes her case to Princess Celestia herself that Dash is the best flyer. Dash meanwhile doesn't make a big deal of it. She's just happy things have worked out. I would imagine that the events have this episode really stand out in Rarity's mind... and especially so since it's not the only time Dash has come flying to her rescue. Both in Secret of my Excess and in Wonderbolts Academy Rarity is again in free fall and gets saved by Dash's well time flying; sur in those cases others are involved, but it's possibly enough to create an association for her. In the most recent episode, Bats!, when Rarity was scared she immediately thinks to join Rainbow Dash ahead of any of the others; it might be she associates Dash with gallantry and with rescuing her from peril, which would certainly be affecting. Rarity would be one of the best at reigning in the wild Dash. Her general confidence in carrying herself just fits well with the more erratic pegasus. She could keep up with Dash, and actually might be the one with the more dominant hoof. I can see this making Dash as a result slip a little into her attached fangirl mode she's prone to with Rarity, which is adorable. They look good next to each together. - Their designs contrast nicely while they both mostly are cooler colors. It's seriously super adorable.