Every believer would like to be part of the kingdom of heaven. Matthew used the term "Kingdom of Heaven" throughout his gospel. It's used in the sense that it's where God lives, his place of residence. So when Jesus says "blessed are the poor in spirit, for theirs is the Kingdom of Heaven" in Matthew 5:3, it's like he is saying God invites those who are poor in spirit to live with him. He is saying my house is your house. Looking at it this way, who would not want to be poor in spirit all the time, not just when we might happen to be.
Right away, however, we see a problem. Doesn't poor in spirit mean to be depressed and isn't rich in spirit a good thing? If so, then we would need to be walking around depressed all the time. However, that's not what poor in spirit means.
First, notice that it's spirit with a small "s". That makes it human spirit, not The Spirit with a capital "S" which is the Holy Spirit. We can be both poor and rich in human spirit, as well as poor and rich in the Holy Spirit. What's more, they are diametrically opposite one another.
Imagine you're standing at the 50-yard line on a football field. The goal line in one direction is the human spirit goal line and the other end is the Holy Spirit goal line. If we were to head toward the human spirit goal line, we would be getting richer in human spirit. The richness end of the human spirit can be described in three words: me, me, and me. Here, we are filled with human pride, being all we can be, and every other cliché that advertisers throw at us when we take hold of the world. Standing in that end-zone, being rich in the Holy Spirit would be furthermost from us, all the way down at the opposite end-zone.
If we were to head that direction, we would be getting poorer in human spirit as we got richer in the Holy Spirit. At the Holy Spirit end of the field, we are prostrate on the ground and all we can say is "Thank you, Jesus." The "me" has totally disappeared and it's all about God and everyone else. We have become completely humble and that's what being poor in human spirit means ... total humility. We are fulfilling Jesus's two greatest commandments: "You shall love the Lord your God with all your heart, and with all your soul, and with all your mind. This is the great and first commandment. And a second is like it, You shall love your neighbor as yourself" (Matthew 22:37-39).
Obviously we want to reach that end-zone, but true humility is hard to come by. We throw that word around sometimes in meaningless ways almost as badly as we do the word love. But the Bible gives us the perfect example of what true humility is. If there is any single person you would not set above you and wash their feet, how humble would you be?
That's exactly what Jesus did as described in John 13:1-17. Peter balked at having his feet washed, probably because he thought it too lowly a thing to do, certainly nothing he would ever do and so something he would never allow Jesus to do. When he did so, Jesus told him, "... you have no part in me."
Jesus washed feet as an example to us all. Only when you can look at every other brother or sister in Christ as being better than you, and willing to wash their feet, then you have reached that point of being poor in spirit.
Let nothing be done through selfish ambition or conceit, but in lowliness of mind let each esteem others better than himself. Let each of you look out not only for his own interests, but also for the interests of others. Let this mind be in you which was also in Christ Jesus, – Philippians 2:3-5