If you speak in sententious phrases, your listeners are probably falling asleep, as your speech is pompous and pretentious, and full of moralistic babble.
When sententious first appeared in English — back in the late Middle Ages — it meant "full of wisdom," but now it usually has a negative sense, meaning heavy handed and self-important. The sententious blowhard makes people laugh, and you can probably think of at least three cartoon characters who fit the bill — often a politician or minister who drones on and on, oblivious to the fact that his audience is snickering or trying to sneak out.