Rainbow Dash was a bit surprised to have become such good friends with an Egghead like Twilight Sparkle. It’s not that she thought nerds were morons or menaces to society or anything, she just never really had that much of a connection with them.
From the day she met Twilight, she had a feeling she’d be an interesting pony to be around, and she was correct when she thought that as Twilight had always been fun to be around in a way, there was more to her than just being a know it all. She was kind, loyal, treated every creature with respect they deserve, and passionate about everyone and everything she cares about.
She didn’t really understand her fascination with books at first, in fact she found the mere idea of reading to be undeniably, unquestionably, uncool but after Twilight had introduced the Daring Do series to her when she was in the hospital, she realized that everyone has a different type of book they like and they should at least try to read before rejecting it, Twilight taught her that.
She also appreciated how much Twilight was willing to do for her friends, for example, when Twilight was willing to try and teach Rainbow Dash about the Wonderbolts in her own way, despite the fact that she didn’t really bother to pay attention to the whole thing, and even blamed Twilight for how she was wasn’t learning anything. Looking back, she should have been more grateful about what she was trying to do.
It was also really considerate of Twilight to continue to try helping Rainbow Dash pass the test despite all that, some ponies would probably have given up but she didn’t, and that meant a lot to her. She was able to achieve her dream and join the Wonderbolt Reserves and later a Wonderbolt, she really owed that success to Twilight. She doubted she could have done it without her.
Twilight was always around to keep her grounded, and make sure she didn’t get too out of control with her ego and recklessness. She was a pretty positive influence in her life, sure Applejack also played a similar role but she was probably less patient with her than Twilight was.
She also appreciated how Twilight can sometimes teach her lessons without her even knowing it. Like the time where she had Rainbow Dash coach the cheerleaders at the Buckball Game, showing that you don’t have to care about something other creatures like as long as you care about the ponies who like it.
She hated to admit it, but there was probably no way she could have learned that lesson without Twilight tricking her into learning it, and it was a valuable one too.
At the end of the day, Rainbow Dash will always appreciate Twilight for her patience, compassion, and ability to make Rainbow a better pony. That was something she would always be thankful for.