I think it might be due to the type of problem we are facing as living entities. We have a consistent never ending goal of "not killing ourselves" and "not mucking up our chances of reproduction". Pain is one of the signs that we might be near doing these things. Every day we manage not to do these things is in some way a good day. This presents a baseline of utility where anything less than it is considered negative and anything more than that positive. So it just might be what this type of algorithm feels like from the inside.