Because he's arrogant and he wants to drag everybody down below him as he hates being looked down upon. As a result, he insults everyone. Also, he doesn't really curse. Japan doesn't really have curse words, like what we use. Its more like rude variations of words. "Teme" is often translated as "bast**d", but really it's just a rude way of saying "you." So, Nnoitra is very disrespectful, but he doesn't really curse.