Simply put, a Patronus is an immensely difficult to cast, powerful magical protective shield.
A witch or wizard uses the Patronus Charm to create a Patronus by saying the incantation “Expecto Patronum” while concentrating on their happiest thoughts and memories.
If performed correctly, this spell projects the caster’s happiness into a shielding guardian against Azkaban’s Dementors.
Why Did Snape and Lily Have the Same Patronus?

Snape was most likely aware that Lily’s Patronus was in fact a doe. Snape loved Lily Potter with all his heart for all his life.
These deep, unchanging, and all-consuming feelings caused Severus Snape’s Patronus to take on the exact same form as Lily Potter’s Patronus.
Anyone who has read the Harry Potter books or watched the movies will remember the chapter -or scene- revealing Snape’s lifelong, undying love for Lily which was revealed by Snape casting his doe-shaped Patronus.
And, since Patronuses are a pure, magical, and concentrated form of hope and happiness, Snape’s Patronus taking on the shape of his eternal, one true love (at least from his point of view) starts to make perfect sense.
This rings especially true if we look at the little we know about Severus Snape’s life; we saw no joy or hope from his unhappy childhood to his seemingly disgraceful death.
In fact, Harry sums it up quite nicely when he explains it to Voldemort:
‘Snape’s Patronus was a doe,’ said Harry, ‘the same as my mother’s, because he loved her for nearly all of his life, from the time when they were children.’
Harry Potter and the Deathly Hallows
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What Is the Significance of Severus Snape’s Patronus?
JK Rowling shared with fans during a 2007 live chat that Severus Snape was the only Death Eater who could cast a Patronus due to his deep love for Lily Potter.
The other Death Eaters had no need to learn such a spell. They usually found themselves fighting alongside (not against) such foul creatures as Dementors.
The ability to cast a corporeal Patronus, in that it takes on a distinct shape instead of fog or vapor, is considered the mark of a highly accomplished witch or wizard.
Severus Snape, therefore, was a highly skilled wizard who still had enough good and benevolence within him to create the doe-shaped Patronus.
Did Lily Know Snape Loved Her?
It seems very unlikely that Lily knew Severus’ true feelings towards her. There is no mention in any of the Harry Potter books that Severus ever admitted to his feelings, in fact, he insisted that they were friends.
Snape and Lily had been childhood friends who grew further and further apart in their adolescent years until the fateful day that Severus slipped and called Lily the unforgivable insult “mudblood”.
While Severus seems to have fallen for Lily quickly and hard, Lily approached their friendship cautiously and seems to have been mostly drawn to Snape because of his knowledge of the (to her) unknown magical world.
Why Did Snape Call Lily a Mudblood?
Snape called Lily a Mudblood in reaction to being taunted and humiliated by James Potter while other students watched and cheered James on. Lily’s intervention made him feel even more helpless and weak, causing him to slip.
Snape’s self-restraint was exhausted for one brief moment in response to James’ bullying, and it was then that he uttered that fateful insult that he could never take back– irrevocably damaging their relationship.
Did Lily Love Snape Back?

No, we can be almost certain that Lily Potter did not harbor romantic feelings toward Snape. Her love for Snape was completely platonic. Severus was a childhood friend whom she grew more and more apart from due to his affiliation with the Dark Arts. Lily had no romantic feelings towards Severus Snape.
In the same live chat as mentioned above, Rowling also shared that Lily could have grown to love Snape romantically if he had chosen a different path, making it clear that Lily did not love Snape back.
Their friendship started with a mutual need for each other-
Lily both appreciated and relied upon Snape’s knowledge of the magical world and how to navigate it. Snape had no friends or confidants and no escape from his unhappy childhood home besides Lily.
Severus was immediately infatuated with the stubborn and strong-willed (yet at the same time immensely kind) Lily. While Severus’ affection quickly grew into attraction, Lily’s feelings for Snape remained platonic throughout their friendship.
Who Loved Lily More Snape or James Potter?
Both loved Lily in different ways. James’ love for Lily was pure and genuine. In contrast, Severus’ love may have started out that way but grew into an obsessive, possessive, and all-consuming love.
This fact becomes especially obvious when Lily tells Severus that he is ungrateful towards James in “The Princes Tale,” and he replies that he won’t let Lily think of James as a hero, losing his temper.
On the other hand, James once and for all proves his selfless love for his wife and son in his last action, when he sacrifices himself trying to stop Voldemort.
Why Does Harry Ask Dumbledore About Snape’s Patronus?

Curiously, Severus’ Patronus mimicked Lily’s, whereas James’ Patronus complemented and completed hers. This beautifully illustrates the difference between James’ genuine and Severus’ obsessive love for Lily.
Given the fact that his father, who also deeply loved and cherished Lily, had a different Patronus, it is understandable that he wanted to know Dumbledore’s thoughts on the matter.
Harry understood that Snape’s love for Lily was responsible for his Patronus taking on the doe form.
Snape himself made this clear during the heartbreaking conversation he has with Dumbledore when learns that, in the end, Harry will have to die to defeat Voldemort.
“But this is touching, Severus,” said Dumbledore seriously. “Have you grown to care for the boy, after all?”
“For him?” shouted Snape. “Expecto Patronum!”
“After all this time?”
“Always,” said Snape.
Harry Potter and the Deathly Hallows
Learn more about the “Always” quote meaning.
Did Snape Love Harry?
Snape did not love Harry. He was not only a spitting image of his mortal rival, but his very existence was a constant reminder of Snape’s unrequited love for Lily. However, it was also this same love for Lily that Snape chose to defend and protect Harry as well.
Snape was incapable of seeing beyond Harry’s appearance to notice that there was a lot of Lily in him, so he continued to loathe Harry for the rest of his life.