Open the combination locks and rolled-up doors and reveal a secret tucked away: self-storage is one of America's fastest-growing industries with nearly $40 billion in annual sales - a number that looks set to continue it's upward trajectory.Americans possess a lot of belongings that spark joy and for which they are not quite ready to discard - whether experiencing a downsizing (retirement or job loss), the merging or splitting of households (marriage or divorce), relocation (military deployment or college), or any variety of other reasons.