Due to Celestia’s and Twilight’s busy schedule, neither was able to be with their so very often. To make sure his strange bouts of magic were kept under control, they asked Discord to keep the foal company and train him in using his unique brand of magic. Sirius adored his mentor had lots of fun learning magic from the god of chaos.
 
An interesting quirk of Sirius is his slightly chaotic magic. Due to his mothers previous encounter with being discorded, her son was born with a slight twinge of chaos, hence their choice in having Discord teach him.
 
Truly, Discord loves the foal but feels uncomfortable with how often he finds the foal talking to himself and mentions stars as friends.