"Poor in spirit" does not mean to conduct one's life without vitality, nor does it mean that a person is weak. Would we ever accuse Jesus of being weak? Jesus was the personification of humility. People think of humility as weakness because they are judging carnally by man's spirit, by sight. But the Spirit of God, the faith of God, judges according to things not seen—the Kingdom's standards.
Here is a definition of poor in spirit from a commentary by Emmet Fox on the Sermon on the Mount:
To be poor in spirit means to have emptied yourself of all desire to exercise personal self-will and what is just as important to have renounced all preconceived opinions [prejudices] in the wholehearted search for God. It means to be willing to set aside your present habits of thought, your present views and prejudices, your present way of life, if necessary, to jettison in fact anything and everything that can stand in the way of your finding God.
When Jesus counseled us in Matthew 18:4 that unless we became as little children, we would not even be in the Kingdom of Heaven, He was not holding up a child's innocence or purity as a model. He was not counseling us to become childish but to have a child's unconcern for social status, honor, or anything similar. When we are carnal, pride is such a master that we have little choice but to follow it. It is plowing the way before us. One who is truly poor in spirit, however, can ignore pride and follow God's lead.