Good question. The test of time. Humans are so unpredicatable and you don't know what's in their minds. Some wear masks and underneath are scheming for their own amusement, selfishness, or whatever reasons. For example pedophiles are lurking and waiting to amuse themselves with our children.
Animals on the other hand are predicatable, they are who they are. We know what to expect from them. A snake is a snake. For example, elephants take care of their young and should a parent die, the herd takes care of the young. They don't leave them behind. They have instincts that keep them together as a caring herd. They sacrafice for their young. Just what humans are supposed to do.