During Hearts and Hooves preparations,

Princess Cadance and Spike are accidentally slipped a Love Potion, by a waitress who intended to give it to some-pony else, causing Spike to become infatuated with Twilight, and Cadance falls for the waitresses Dream Beau.

LESSONS:

1: Some can't help who they love or who they wish to love and have.

2: You cannot change how someone feels simply because you want them to.