Because we love flying! We're prepared to accept a lower wage than we could get in other careers in order to do something which we love.
Certainly if the previous paragraph doesn't apply to you, you should look at a career as a train driver,
Just to clarify that i am too old now for a flying career and also couldn't afford the drop in pay for the kind of job i would get if i went commercial now. 37yrs old low hours pilot. Not exactly going to walk into a rhs on a 737 etc now am i
