What is the relationship between Snape's Patronus and Lily?

Question

When I read the Harry Potter series I found this part confusing! Is there a relationship between Lily Evans's and Snape's patronus?!

Answer 1

Snape's Patronus was the same as Lily's because he was in love with her.

A wizard's (or witch's) Patronus can change shape to match the Patronus of someone they're truly in love with. The feeling of love does not have to be mutual for the Patronus to change shape. As revealed in Snape's memories during Harry Potter and the Deathly Hallows, he was in love with Lily since they were young children, before they even started Hogwarts.

Harry explains this to the Dark Lord here:

“Severus Snape wasn’t yours,’ said Harry. ‘Snape was Dumbledore’s, Dumbledore’s from the moment you started hunting down my mother. And you never realised it, because of the thing you can’t understand. You never saw Snape cast a Patronus, did you, Riddle?”

Voldemort did not answer. They continued to circle each other like wolves about to tear each other apart.

“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. You should have realised,’ he said, as he saw Voldemort’s nostrils flare, ‘he asked you to spare her life, didn’t he?”

The same thing happened to Tonks when she fell in love with Remus Lupin and he didn't seem to be returning her feelings.

Not knowing he was the reason for it, Harry asked Lupin why someone's Patronus could change.

“He was, however, reminded of a question he had with regard to Tonks, and who better to ask than Lupin, the man who knew all about Patronuses?

‘Tonks’s Patronus has changed its form,’ he told him. ‘Snape said so, anyway. I didn’t know that could happen. Why would your Patronus change?’

Lupin took his time chewing his turkey and swallowing before saying slowly, ‘Sometimes … a great shock … an emotional upheaval …’

‘It looked big, and it had four legs,’ said Harry, struck by a sudden thought and lowering his voice. ‘Hey … it couldn’t be –?” - Harry Potter and the Order of the Phoenix, Chapter 16 (A Very Frosty Christmas)

He later learned the reason, when witnessing a conversation between the two:

“You see!’ said a strained voice. Tonks was glaring at Lupin. ‘She still wants to marry him, even though he’s been bitten! She doesn’t care!’

‘It’s different,’ said Lupin, barely moving his lips and looking suddenly tense. ‘Bill will not be a full werewolf. The cases are completely –’

‘But I don’t care either, I don’t care!’ said Tonks, seizing the front of Lupin’s robes and shaking them. ‘I’ve told you a million times …’

And the meaning of Tonks’s Patronus and her mouse-coloured hair, and the reason she had come running to find Dumbledore when she had heard a rumour someone had been attacked by Greyback, all suddenly became clear to Harry; it had not been Sirius that Tonks had fallen in love with after all …

‘And I’ve told you a million times,’ said Lupin, refusing to meet her eyes, staring at the floor, ‘that I am too old for you, too poor … too dangerous …’

‘I’ve said all along you’re taking a ridiculous line on this, Remus,’ said Mrs Weasley over Fleur’s shoulder as she patted her on the back.

‘I am not being ridiculous,’ said Lupin steadily. ‘Tonks deserves somebody young and whole.” - Harry Potter and the Order of the Phoenix, Chapter 29 (The Phoenix Lament)