By casting the patronus, Snape indirectly answered the headmaster that he still loved Lily. That all he did was for Lily’s sake and her death shocked him so much that his patronus took the form of Lilys’, whom he loved dearly.
Severus Snape’s Patronus was also a doe, which symbolized his love for Lily. Snape uses his doe Patronus to show Dumbledore that he never fell out of love with Lily, his childhood best friend. According to Rowling, Snape was the only Death Eater who could conjure a Patronus charm at all.