This teaching from the Beatitudes has always been difficult for me to understand, yet in this reflection from the wonderful book by Reverend Gerald Vann, O.P., The Divine Pity: A Study in the Social Implications of the Beatitudes, it is summed up nicely.
“You are only a steward of all that you have and all that you are; a steward for God and for His family. But you are meant to be more than stewards of God’s things: you are meant to be lovers as well. If you hurt anything of the things that God has made—by lust, or tyranny, or blindness, or by using things in any way as mere means to your pleasure or profit—you hurt yourself and all the world, because to that extant you continue to destroy the unity of the family. But if you love, and therefore can serve as well as use, can reverence as well as master; if you are a contemplative and have learnt to see and love instead of grabbing, and if your love is worship of God and not of yourself, if your love is as deep as the sea but as carefree as the wind, then you return to the integrity of God’s family, and, having nothing, you are at peace, because you have nothing to lose, and at the same time you possess all things, for yours is the kingdom.” (p. 41)