Dude, we don't upvote anything, we just really love the choice of beans.

The reason I say we can "prove" our consciousness is not because we can prove it to others, I mean we can prove it to ourselves because we can experience it. Anything outside our current consciousness, however, is not available with such ready belief. For everything else we must be taught to trust that it is true, even if it is impossible to have absolute proof of it.

I think the main reason of the "default assumption" that only humans have consciousness is that we ourselves are human, and we are only able to prove our own consciousness. Culturally, at least in the Western worldview, humans are framed as equal. This means that we feel comfortable saying that other humans have consciousness as well, but start to get a little antsy about saying that "lesser" creatures have that same experience.