In which situations does a wizard's patronus change due to love?

Question

My friend and I were talking about this. The only two instances mentioned in which patronuses change into a loved one's patronus, is when the love is unrequited (Snape to Lily, Tonks to Lupin). My friend thinks a wizard's patronus only changes with unrequited love, while I think that is too specific and weird.

"You'd better put that cloak back on, and we can walk up to the school" said Tonks, still unsmiling. As Harry swung the cloak back over himself, she waved her wand; an immense silvery four-legged creature erupted from it and streaked off into the darkness.
"Was that a Patronus?" asked Harry, who had seen Dumbledore send messages like this. "Yes, I'm sending word to the castle that I've got you or they'll worry. Come on, we'd better not dawdle."

_{The Half-Blood Pricne - page 157 & 158 - Bloomsbury - chapter 8, Snape Victorious}

"Hagrid was late for the start-of-term feast, just like Potter here, so I took it instead. And incidentally," said Snape, standing back to allow Harry to pass him, "I was interested to see your new Patronus."

_{The Half-Blood Pricne - page 160 - Bloomsbury - chapter 8, Snape Victorious}

"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-colored hair, and the reason she had come running to Dumbledore when she had heard a rumour someone had been attracked by Greyback, all suddenly became clear to Harry; it had not been Sirius that Tonks had fallen in love with after all.

_{The Half-Blood Prince - page 624 - Bloomsbury - chapter 29, The Pheonix Lament}

And...

"But this is touching, Severus," said Dumbledore seriously. "Have you grown to care for the boy, after all?"
"For him?" shouted Snape. "Expecto Patronum!"
From the tip of his wand burst the silver doe: She landed on the office floor, bounded once across the office, and soared out of the window.

_{Deathly Hallows - page 687 - Bloomsbury - chapter 33, The Prince's Tale}

But if it changed for any love, wouldn't that mean that couples would end up with the same patronuses? For example, if Lily fell in love with James, couldn't her patronus become a stag, which in turn would make Snape's a stag, too? And is it only romantic love? Does a parent's patronus change to their child's? But then that seems way too general. Everyone would have the same patronus.

Is this only strong, passionate love? But how would that be judged? That would put down all other kinds of love.

(edit: I know Lupin did love Tonks back, but as DVK said below, he was still rejecting her)

Answer 1

I think there are two cases you have forgotten to mention; that might help understand the whole patronus debate a bit better. The first one is with James and Lily and the fact that their patronuses, albeit not the same; but a doe and a stag. I think that it is more than just a coincidence that their patronuses are the same as each others but with different genders. Where as Severus' patronus mimics Lily's; her's and James' match as if it was the way fate intended them to be together; each others soulmate.

Then there is the case of Harry's patronus which is in the form of a stag, like James'. This brings me onto what I think is the explanation about what shape a patronus forms. When Harry first produced his patronus, he wanted nothing more than for his Dad to come out, to be able to see his Dad, because he believed in his heart that his Dad had been the one to save him from the dementors in that clearing. And when he looked in the Mirror of Erised, it showed him that his deepest desire was to be with James and Lily and the rest of Potter clan. Hence why I think that patronuses form to imitate the one thing a person desires more than anything else.

Severus desired to be with Lily, which is why his became a doe, to imitate hers. James Lily desired to be together, so theirs became a Stag and a Doe; a perfect match, two beings that belonged together. Harry desired to be with his parents; so his patronus became a Stag; just like his father's, and the male version of his mother's. Tonks desired to be with Lupin and for him to see that she accepted him, inner wolf and all; so her took the form of a wolf. Another example we can think about is Albus Dumbledore's patronus. I am fairly sure his patronus was a pheonix. A pheonix rises from the ashes and helps those who are truly in need, and heal those who require it. And I like to think that is what Albus desired to be able to do. To be able to help others, to rise from the ashes; figuratively speaking; when all hope seems to be lost and help out. He desired to give help to all those who needed it; which is why his patronus was in the form of a pheonix.

Now to get to the question in hand, why a patronus might change. In Severus' cases I don't actually think that his patronus did change, but merely took the form of Lily's from his very first go. I put this down to the fact that Severus learnt to love Lily from a very young age, way before he would have been able to produce a patronus. It is even implied that 13 is too young for a patronus to be produced, and that even the majority of older wizards can't produce a corporeal patronus, which is why it was so surprising to see Harry manage to produce one. However we can not forget that there are others in the books that manage to form a corporeal patronus without any real significance to any desire they might hold. I think that a patronus takes the form of a spirit animal of the person casting it; an animal reflecting the persons inner personality, and only changes when the person realises their truest desire. That is why Tonk's changed, as she did not realise he desire until much later in life; after she had managed to cast a corporeal patronus, which is why her patronus changed.

I also think it is wise to point out that those who are an Animagus; they more often than not take the form of the Animagus. I.e, James Potter and Minerva McGonagall. And also people may argue that love and desire are basically the same thing; to which I cannot deny. So take from what I personally think as you will. Afterall, this is just my take.

Answer 2

It seems unlikely that just falling in love with someone is insufficient to change one's patronus.

There was no mention of Harry/Ginny or Ron/Hermione experiencing such a change - something we would have seen considering how central these characters were - and yet it is entirely clear that the members of these two couples love each other very much.

So, it clearly takes some additional element to affect the change. It also seems like it would be inconsistent with other aspects of JKR's world for it to be something simple like "unrequited love".

My theory is that the changing of one's patronus is probably an unconscious attempt to show the person how much you love them under exceptional circumstances.

In Snape's case, he has loved Lily for most of his life. She was the first person to accept and befriend him. I think it would be exceptionally difficult to quantify the depth of feeling this would create. Then circumstances created a rift between them, one that would never be mended.

The case of Tonks and Lupin is nearly as tragic. The level of distrust and prejudice that werewolves appear to suffer in magical society is exceptional. It would be difficult to live with this kind of abuse as long as Lupin has without it strongly affecting your opinion of yourself. As Tonks tries time and again to break through the walls that Lupin has erected around his heart to protect himself, she is forced to resort to extraordinary measures.

Answer 3

If your patronus changes cause you love someone, if you fall out of love will it stay the same or change to something else?

J.K. Rowling: Your Patronus only changes if it's eternal love, unchanging - part of you forever.

_Twitter

Answer 4

In addition to these excellent answers, I would add that in order to produce a Patronus, one must think happy thoughts. In other words, a Patronus is a physical manifestation of your happiness, and that is what drives the dementors away (because dementors feed off negative emotions like sadness and despair). I think Tonks's Patronus changed because it was Lupin that was making her happy; the happy memories she clung to in order to make the Patronus had changed to thoughts of Remus.