We all know McGonagall was a stern, motherly figure, and she loved her kids to death, especially Harry, Ron, and Hermione. We also know she was an animagus who transformed into a cat.
SHE IS THE MOTHER CAT
HER PATRONUS IS HER “KITTENS,” HER “CHILDREN”
HER PATRONUS IS HARRY, RON, AND HERMIONE
Because McGonagall is that badass motherly professor that loves her kids to death so much that taking care of them and watching them grow is the one thing that makes her absolutely happiest in the entire wizarding world.