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.