what is the psychology behind people sympathetic to animals but not humans?

Animals love you even if you're unkind/mean-spirited/cruel. People don't. Animals don't talk back, tell you you're wrong or leave you. I suppose such people are out to protect themselves from rejection/betrayal Happy Monday to you! :)

They've been hurt by humans so they anthropomorphise animals? The unconditional love that animals appear to give? Animals don't talk back.

I like you question. I've thought about this before. I am a member of the Yancey County Humane Society up near Burnsville, N.C. even though our home is in North-Central Florida. Earlier this year we adopted two kittens from the shelter west of town when we were up there. I put my money where my mouth is and when my kids were young they did community service work at the shelter (as a part of their educational requirements).

I think I would slightly amend your notion - at least as it applies to me. I am more sympathetic to animals than I am adult humans. For me, the reason is obvious. Most of the adult humans I know have at least the capacity to be resourceful enough to rebound from setbacks. Of course I know there are the extreme cases as in natural disasters, etc. I see adult humans as having had some say in their fate or at least some options in life. Whereas I see animals, particularly domestic animals, as being pretty much dependent upon us.

This, by the way, is the reason why I defend the rights of the unborn and other children with more vigor than I defend the rights of grownups. Most grownups I know have had some choices and can fight adversity but the unborn and little ones are pretty much totally at our mercy. People who are cruel to animals and/or children are, in my opinion, the lowest of the low.

Animals lie by you and cuddle when you are sick, lick your tears when you cry, don't mind when you call them silly names and make up songs about them, and generally don't judge you. None of my pets have ever questioned my word, insulted me, nor have they accused me of any wrongdoing. They also don't care that I'm on the computer so much.

These people are generally referred to as animal rights activitists. Many of them work in the federal government as evidenced by a large group of California farmers who can't get water as that might endanger some insect or fish no one's ever heard of. Remember the snail darter? A little fish you might refer to as a minnow? Same group of people. But the psychology behind how these folks think is probably quite interesting and alien to the way you do.