What makes a circuit breaker go bad?

A circuit breaker can go bad from being tripped too many times. Many people don't understand that the tripping of a circuit breaker indicates a problem that needs to be corrected. They usually just reset the circuit breaker, leading to a very common second (or third, or fourth) trip. Circuit breakers tripping are for the prevention of fire due to excessive heat in the circuit. They're not supposed to be tripped repeatedly. This can wear the breaker out. Believe it or not, I've also seen circuit breakers fail to re-energize after being turned off. I speculate this was actually caused by the breaker never having been cycled (it was a main breaker), and the time elapsed since it was installed. Electrical equipment doesn't last forever. It's the same as anything else.