Most animals don't have self-awareness, though some even have that, like great apes, certain cetaceans, elephants, and possibly even some birds.
The only things that seem to be uniquely human are complex language, cultural evolution and prolonged neuroplasticity during childhood and early adulthood.
Even in humans the state of being animals that possess consciousness varies over the course of time: new-born infants are in a different state of consciousness than 4 year old children, for example. Not to mention our regular fading in- and out and transitioning between various states of consciousness during our sleep cycle.
The first important step towards a better understanding that would allow proper assessment would be to develop a sound metric to allow qualifying consciousness. Doesn't have to be precise, but a scale from say 0 (non-conscious) to 100 (awake neuro-typical sober human adult) would be a great step forward IMHO.
Maybe we are defining consciousness differently but how do you know? how do you prove that? Don't get me wrong I too believe that animals have consciousness, but I think humans other than me have consciousness too and I can't prove that either. That's a big part of the whole issue particularly in regards to whether the current ai of the week is conscious or not.
You can demonstrate that animal and human brains achieve similar brain states given similar stimuli but how do you demonstrate that those brain states are sufficient for/require consciousness? for all we know every animal is a philosophical zombie and we can't prove otherwise.
Its true we have additional brain structures responsible for higher reasoning and linguistic abilities that other animals don’t share, but it seems likely that these features are layered on top of those other capabilities we inherited from our common ancestors with other mammals.
In support of this, there are some behaviours we share with other animals that are not conscious, or at least that are so automatic that we are essentially mere observers of our own behaviour. This includes many instinctive behaviours, and these are often shared with lower order animals that do not display sophisticated awareness of their own existence and that of others. It seems reasonable that we inherited those behaviours from common ancestors with such animals (lizards, frogs, etc) before self consciousness evolved.
You want arguments supporting that humans dislike seeing animals suffer? To most people that would be obvious, I'm not sure what to say.
And the fact that I don't get downvoted here is evidence people here also think that animals are conscious, which supports my argument that people think animals are conscious.
Most humans see animals as having a consciousness with feelings and dreams like us. Why would you think otherwise? Why else would it be illegal to torture animals? That we even call it "torture" means we think the animals suffers from it, we don't say we torture a rock when we crack it, that means we see them as having a consciousness.