Why do people ignore fire alarms?

A lot of people assume when the fire alarm goes off that there isn't a fire. This may be because they are frequently set off for maney different reasons (such as smashing a call point for a joke) which is why that is dangerous. One day there will be a fire and someone will ignore it for that reason. Other times it may be just a general test or service that they have been informed about. Or once they have had full conformation that there isn't a fire then they stay put.