It's like this.
You're a real tough guy, right? Well, imagine you have a smaller friend, and your friend was getting bullied by this other guy. And you said, hey, go fight that other guy, I'll jump in and help you out if he pulls any dirty shit.
So, your small friend goes and starts a fight with the mean bully, knowing that you have his back. Then, the bully pulls out brass knuckles. You see this happen and have plenty of time to step in, but you don't. The bully proceeds to beat the living shit out of your friend, while you stand by and watch.
The whole neighborhood sees this happen. Are they gonna trust you the next time you promise them anything?