Sometimes for good reasons. Some of the processes that underlie pain, sensation and coping with environmental challenges are very conservative in evolutionary terms, and although our cognitive abilities must have increased enormously over the last two million years, other aspects of our behavior are probably very much the same as those of other animals. Of course, I could say to you that I only know about my own awareness, and I shall never have access to your awareness or your capacity to suffer. But that kind of solipsism would be rather foolish. If I specify the criteria that lead me to such an inference, I can apply them to other animals.
People do that intuitively.
But sometimes it is rational to treat animals as though they had some human properties. I know you've been concerned with how animals and humans become what they are and how their behavior develops, and that's been the basis of a lot of your research. You seem to be able to describe in understandable language processes which many people find very complicated. Some people see the process of growth and development as very simple. They seem to think it is read out of the genes, and that when the human genome project is completed we shall have the Book of Life, which will give us a complete an understanding of all human nature.