All things belong to God. Think about it: what can be considered yours alone? Even yourself belong to God, for no one comes into the world on his own, knowing where and when he was to be born. Everything you have now was already here or made from things already existing. Not a thing has your name stamped on it that says it's yours forever. We'll all be gone soon and not be able to hold onto any material things. Even non-material things such as love, appreciation of natural beauty, etc., belong to God, for he made us in his image in the first place. When you are aware of this truth, you become immediately liberated!!! There's no need to be attached to things that are transient, to fight for things that cannot truly be yours anyway, or to get uptight about not being able to get something you want. You will always have God's love and that is more than sufficient.
A footnote: All things include the people you know.