There are very clear anatomical differences and similarities among and between various groups of animals. Mammals have brain parts that are non-existent in other animals. Mammals have emotions, and feel pain. It's a pretty sure bet that none contemplate and agonize over these things the way humans do, although the great apes probably come close.

"Reptilian" type brains, found in reptiles, amphibians, and birds, do not have emotions and can't process "pain." These animals mostly function on instinct and reflex. Humans have tended to anthropomorphize their observations of animals, ascribing concepts such as fear and love to actions that are really based on much simpler brain processes. When you get to the "lower" life forms, it's all pretty much reaction to stimuli. A lobster does not suffer when plunged into boiling water. It reacts to the heat stimulus in ways programmed to get it away from danger. It does not "think" of danger, nor "wish" to avoid it. It is human nature to look at the process as if we were in that position and ascribe our feelings to the lobster, but they don't apply.

This is all pretty basic stuff that can be gleaned from a large amount of objective research out there. Humans are far and away the most neurologically developed, elevated if you will, creatures on the planet. That's why we are able to study these things. And agonize about them. And learn that not all animals have feelings.