When Rainbow Dash gets crowned a princess for being too awesome (and also because she saved Princess Celestia's life), she thought life couldn't get any better.

Then she's told that, by law, she needs to find a consort in order to become a fully-fledged princess. No biggie. Easy solution: just blackmail somepony into pretending to be her consort. And there's nopony easier to blackmail than Spike.

Alicorn SpikeDash because it needed to be done.