We're not, there are many other species that are sentient. Certain birds exhibit the signs of sentience. Many mammals, including the great apes, pigs, dogs, whales, dolphins, and manatees can be self-aware. It has been suggested that even squid and octopi can be self-aware, although it manifests itself a bit differently than mammalian self-awareness. It is suspected that humans became better at language than other species (whales can rhyme and they give each other names, but don't have languages at human level), which allowed us to conceptualize at a much higher level and conceptualization lead to higher creativity, and our opposable thumbs allowed us to develop tools in a way other non-primates couldn't