Patrono de Snape era o mesmo que o de Lily, porque ele estava apaixonado por ela.
O Patrono de um mago (ou bruxa) pode mudar de forma para combinar com o Patrono de alguém por quem eles estão verdadeiramente apaixonados. O sentimento de amor não precisa ser mútuo para o Patrono mudar de forma. Conforme revelado nas memórias de Snape durante Harry Potter e as Relíquias da Morte , ele estava apaixonado por Lily desde que eram crianças, antes mesmo de começarem Hogwarts.
Harry explica isso ao Lorde das Trevas aqui:
“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?”
A mesma coisa aconteceu com Tonks quando ela se apaixonou por Remus Lupin e ele não parecia estar retornando seus sentimentos.
Não sabendo que ele era o motivo, Harry perguntou a Lupin por que o Patrono de alguém poderia mudar.
“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)
Mais tarde, ele aprendeu o motivo, ao testemunhar uma conversa entre os dois:
“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)