If you can’t understand this one, you’re pretty stupid… See what I mean? Most of us know better than to call other people names and insult their intelligence.
Being assertive about our opinions and beliefs is fine, but at some point, assertiveness crosses over into bluntness. And that means you haven’t taken the other person’s feelings into account.
Find out if bluntness is a problem for you. To do that simply ask five of the people closest to you at home and at work something like: “Do I come across as too blunt sometimes?” “Do I say things that hurt other people’s feelings without realizing it?” If you get back some “yes” answers, then you need to pay attention to the ways you communicate.