As far as wolves becoming humans:
Yeah, thumbs are useful, and there are many items in a human city that a wolf would probably appreciate. But, quite frankly, I just don't think a wolf could handle being a human. In the event a wolf wanted to go to a grocery store and buy a steak as opposed to killing a rabbit, here is a sample list of things they would have to learn to do:
-walk upright
-put on clothes properly
-human body language
-differences in cultural behavior b/w humans and wolves (i.e. they sniff each other, we shake hands)
-speak english
-read
-understand the system of human currency
Those are just a few of the things a wolf might need to know how to do if they were to go to a store and buy something. Let alone having an entire alternate life as a human! The human world is ridiculously complex, and proper socialization at the proper age is necessary to be able to operate in our world. I will use feral children as an example. Read this, and if you want, peruse the rest of the site as well.
http://www.feralchildren.com/en/showchild.php?ch=kamala
Kamala did make slow, slow, progress. But her lack of socialization prevented her from ever becoming a normal human. In the event that she had lived longer, she might one day have been normal, but I doubt it. I would also like to reiterate the fact that, she thought she WAS A WOLF. So we can use her as a reasonable comparison to a real wolf.
So. Assume a wolf gains the power to become human. It will take them many, many years to be able to be a normal human. Look at Kamala. She never became normal, and (during her first couple years, at least) probably would have sprung at the chance to return to the forest at any time. So if we have a wolf, who is in the human world, and in a human body, don't you think they would most likely choose to return to their wolf form and to the forest? Kamala certainly seemed to want to.
So anyway, that is my point. Sure, there are conveniences in the human world that you couldn't get in the wild, but being able to learn to be a normal human would take far too long, and I would think most wolves would prefer their natural lifestyle. Socialization is far more important than many people think. Ever met a home-schooled kid who didn't have much interaction with others at a young age? Most people think they're weird. If we think that one of those people is weird, imagine what we would think of someone who was raised as a wolf. Sure, they might, on occasion, turn into a human while in the forest for s*** and giggles, but I doubt they'd ever be able to deal with the complex interactions of the human world. Also, gaining the ability to become a human wouldn't necessarily give them the human lifespan. So, unless they become human and stay that way forever (and even that might not do the trick), they'd really only have about 10 years or so to learn to be a human. I don't think they could do it. Even if we decided to be optimistic and say maybe their lives might extend and be maybe 20 years, that still may not be enough.
IF they were BORN as a wolf who could become human, the above thoughts still apply, to a lesser degree. Just like with human babies, a wolf pup wouldn't really be capable of understanding the TF concept (if it could ever understand at all) until it was a little older. Let's say 2 months? So a 2 month old wolf pup becomes human (probably a four year old kid). Unless they stay human, there's still the lifespan problem. If they became human and TFed back to wolf frequently, their human body would age rapidly. About every six months or so, they'd have to learn to interact with a whole new age group. And they'd probably only live to be about 15ish (max), so that's not really much time there.
Anyway, the point I'm trying to make is this: because human social interactions are so complex and difficult to learn for those who have grown up believing they are something else (as evidenced by feral children) I believe that a wolf would just say 'screw it' and go back to wolf, b/c they wouldn't be able to handle being human.
And I really don't think a wolf would be able to ever understand any of it anyway. IMO, our minds are more complex for our more complex lifestyles. You can argue with me on this if you want (I'd really rather not though, that's hardly the point), but there is one thing that really can't be argued. The problems of socialization. Those have been proven in many instances of feral children.
The biggest difference between a feral child and a wolf is choice. Most feral children (those raised by animals, not the isolated kind) are physically prevented from returning to the wild, and thus are forced to learn a handful of words and how to walk upright. However, a wolf with the power to become human has the power to TF back wolf again anytime they want and return to the forest. IMO, they most likely would.
As far as reincarnation goes, I personally don't believe in it. You have a right to believe in it if you want, but you can't exactly use your experiences to prove that something is fact. However, for the sake of this discussion, I will assume that reincarnation exists and that you were a wolf at one point. Despite your past lives, you are a human in your current life and have always been human in this particular life. You were a human baby, and were socialized as a human. Thus, you are capable of handling human society. The wolf in you can appreciate the joys of human life, because the human part of you knows how to handle being a human, and isn't miserable in your human body, I assume.
Anyway, those are my thoughts on why wolves wouldn't want to be humans. I totally rambled up there, and I'm sorry about that. I really should learn not to do that. Anway, for those of you that actually read it, maybe you see where I'm coming from.