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
"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)