It's wrong to break the trust of a good friend.
Tommy told Steve a really funny secret that he didn't want him to tell the rest of his friends.
Steve wants to make his friends laugh.
Steve tells a funny joke he read on the internet the previous night.
Steve tells Tommy's secret to his friends, prompting a lot of laughter from them.
Tommy laughs loudly, along with the rest of their friends.
Tommy feels completely betrayed in Steve, and refuses to ever talk to him again.