Many comedians steal jokes and try to pass them off as their own. People who do this are actually trying to protect their ego to look good for their audiences.
For most people, ego is believed by them to be who they actually are. Your ego can be associated with your social status, your occupation, your personality, and other things that are not permanent. In reality, your ego is just a small part of who you actually are.
Ego in many cases can be detrimental to your well being. For example, if you let your ego take over, you won't take chances that may actually help you. You may never try stand up comedy, you may never ask out someone that can be a great match for you, or you may never take any other risks that may benefit you. All this just because you don't want to look bad.
You may think you know what other people think of you, but you really don't. What you believe someone thinks of you is really all in your imagination.
When a comedian steals jokes, he or she hopes the audience is thinking "This comedian is really funny! What a genius!" When a joke thief gets laughs, they may believe people are actually thinking this. However, keep in mind many of us know Rodney Dangerfield's material. So in reality, some audience members are thinking "This retard has to resort to stealing jokes to get laughs? What a douche!"
If you have to steal jokes to get laughs, you have no business doing stand up comedy. It is much better to bomb many times and learn from it, rather than deceive people into thinking you can actually write. If you are too lazy or unmotivated to write your own stuff, there are other things you can do. Be an actor, be in a cover band, or be President of the United States.