Mary and Joseph were poor in the truest since of the word, but so are we all. Nothing that we have, from the air that we breathe to the sun that warms our face, is ours. Everything we need for existence, everything we have, all of it, belongs to God. God created it, He owns it, and He alone has the right to use it. We are living our lives, quite literally, in borrowed finery. This is the reality of the human condition, the unavoidable reality of being the creature and not the Creator. We didn't make any of what we have, and it is only by His grace that we are allowed to use it.
The primary difference between the rich and the poor isn't what they have or don't have. The primary difference between the rich and the poor is that the poor are under no illusions about what they lack. They know that everything they have comes from God. They know that it isn't by their efforts alone that they have achieved anything. They are intensely aware of their inability to get out of their situation without help.
Those who call themselves rich are sadly deluded. They tell themselves that they are "self-made" men and women, taking credit for their achievements without any thought for all those who provided a helping hand to them along the way. Because they credit themselves with their success, rather than the God who made it all possible, they scorn those who have less than they do, telling themselves that the people who are poor are poor because they lacked the drive to succeed or that they were just lazy. This attitude generates pride, preventing them from seeing the tremendous graces that God pours out on them each and every day, and all of which have gone in to their financial success.
Not all people who have an abundance of money are rich. Some few of these individuals are aware of the fact that they are not the source of what they have, and are generous with it accordingly. They thank God each day for the providence of His hands, and use their prosperity as a vehicle for helping others. These people are poor in spirit, even when their bank accounts may be full. These men and women are shining examples of God's mercy and love, lifting up their brothers and sisters in time of need.
Even though my bank account has never, at any point in my life, gone higher than four figures, I can honestly say that I was rich when I was younger. I never looked at what I had as a gift from God, but as something I had earned or "deserved". I didn't give credit to God for all those people He put into my life at the right time and right place so that I would be educated enough to be able to take advantage of the right opportunities in the work place in order to land the job that earned the money I was making. Yes, much of my success came from decisions I made - but those opportunities would never have been there in the first place if God had not given them to me. I used my money and my possessions and even my talents without any thought of how I might benefit others with them. My only goal in life, at that point in time, was to accumulate more. Truthfully, I looked to money for all the things that I should have been seeking God. I thank God that I now realize the truth. Like Jesus, I was born into this world poor.