Instinct...
I guess I don't see the line as clearly defined as you do. Unless either of us gets reincarnated as a fish...we'll never know.
We would agree that people "feel" these things...and that our close biological relatives probably also do...If you've ever owned a dog, you would conclude that dogs have a lot of "feelings or moods"...
It becomes a question of how far down the species complexity line you are willing to go before you begin to assume that a species isn't capable of this or that. I won't draw that line. Other's draw it very easily at human/non-human.
Time and time again, we keep having to move that line downward.
Did you see that report (ttp://news.bbc.co.uk/1/hi/sci/tech/2983045.stm)that UK scientists determined that fish feel pain when hooked by a fisherman? It caused quite an uproar (mostly among fisherman), who called it ridiculous. To me...it seemed pretty obvious that, of course, fish feel pain. What is amazing to me is the number of people who would refuse to accept even that and the lengths that science has to go to prove that the fish is actually in "pain".
The trouble is that once you accept that animals share similar responses or feelings...you feel compelled to treat them with more respect that we are willing to. It makes everything so easy if we're just "different" from them.