There are some ideas on why animals reject their young in nature and in captivity.

Some refer to survival instincts which claim that an animal will not approach its young if it was tampered by another animal; the reason is that the young may be use as bait. I really do not think this is the main reason because this instinct will be very weak in a polar bear – an animal with no natural enemies or predators.

I remember another explanation which is mainly about cats, in this case the human scent overwhelms the offspring scent and the mother does not recognize the offspring any more.

This as well sounds rather strange , because there are other means of recognition (sound , sight ) and as far as I know most animals have very sensitive smell organs and I really doubt it if some human stench will affect it.

I think this is the best explanation I have found:

There is no real reason for this. Perhaps the "Why?" question is simply not Appropriate in such circumstances. It's sort of like asking: "Why did so-and-so die of cancer." Or "Why did I have an accident in my car?"

The answer for all these questions is: "Things go wrong."

Animal behavior (including human behavior) is just about the most complex thing in the world to understand. For behavior to go well (and we could, no doubt, argue for a long time about what this means), many things must fall into place. Some of these things depend on the genetic program in the animal's DNA. Others are dependent on the environment in which the embryo develops. Still more are dependent on the environment in which the animal grows to adulthood and lives on a daily basis.

It is possible to make great changes in the typical behavior of some animals by changing the conditions under which they are kept. Overcrowding, stress, lack of stimulation, drugs, malnourishment, all can have an effect on the behavior of animals. Disease may also take it's toll on the behavior of an animal. In some cases, disturbed behavior may be the main symptom of certain diseases.