This is the one ship I push with any sort of passion, and it is mainly out of necessity.
Why would I NEED to push TwiDash? Well, I'm pretty sure I could consider both Twilight Sparkle and Rainbow Dash as wife material. However, they're fictional characters, and I don't want to be pushed too far into a world of make believe since I think that would prevent me from finding a girlfriend of my own. So saying, shipping these two would convince my mind that Twilight Sparkle and Rainbow Dash are both taken, thus knocking out two birds with one stone.
Plus, the fact that these are the two most shipped ponies (Rainbow Dash has the most shipping fics, Twilight tied for the most partners with Fluttershy, but gets the tiebreaker with more total shipping fics) is kinda cool in itself.