In the past, Discord helped protect Equestria along with Celestia and Luna. However, they took classes, Discord did not, so he would kinda just chill outside until they came back. It was clear that Dissy and Tia were developing feelings for each other, Luna was just a filly so Celestia knew she couldn’t just run off with Discord every time he tempted her.
 
We all know what happened after that (stone statue, yada, yada)
 
I’ve been wanting to draw these three in their younger years, Discord and Celestia are around teenage years here, and my headcannon says alicorns get late cutiemarks so no CM for Celestia.