The idea of being poor in spirit is the key to all that is to follow in the Beatitudes. I like the note in the Life Application Bible:

• You cannot mourn without appreciating how insufficient you are to handle life in your own strength.
• You cannot be meek unless you know you have needed gentleness yourself.
• You cannot hunger and thirst for righteousness if you proudly think of yourself as already righteous.
• You cannot be merciful without recognizing your own need for mercy.
• You cannot be pure in heart if your heart is full of pride.
• You cannot be a peacemaker if you believe that you are always right.
• You cannot identify with Christ in the face of negative reactions from others without dying to yourself and renouncing your own rights."

All of these beatitudes are rooted in humility, being poor in spirit.