A glass can only hold so much water. No matter how much more you pour, no more will make it into the glass. I equate this to the thought that things can only make you so happy. So if you have some things, buying more things will likely not effect your happiness level much. Where if you stop and look at those things you already had, you may find that you can be quite happy just appreciating what you have.
It also talks about how filling a house with gold and jade breeds only insecurity. It makes sense to me that in theory the man with nothing to lose feels no need to lock his door at night. Having very valuable possessions makes someone fearful of losing them. I used to be that way with diamond jewelry my husband bought me, I was afraid to wear it for fear the chain would break or I would somehow lose this expensive gift. One day my husband asked why I never where a particular piece, so I told him. He wisely responded, "Honey, I did not buy that for you, hoping you would lock it away and never wear it. I'd rather you wear it always and enjoy it, even if one day it does get lost, then it would be worth it because it brought you joy that it never can sitting in a box."
Amazing how much my husband teaches me everyday, I hope it's always that way. I think today I'm going to appreciate everything I have in my life, instead of focusing on the few things that may be absent.