If someone has low self-esteem than they can never be humble. It's a kind of self-control, where you restrain yourself from being consumed by pride. Or maybe that is modesty and the two words just get mixed up. It's like how someone who is weak can never be meek, it requires no self-control or discipline to restrain your wrath or vengeance when you can't even physically assert yourself. Also, can someone be courageous if they don't know fear? Overcoming fear is what makes courage so inspiring.