First, owning things makes me feel a little better. I had a small hut, then a bigger one, then a cement house. In that process I feel a little better each time. Since no one can peer into my mind no one can know why. But let us say it does. This is all for argument's sake. In reality I do not own a house of any kind. But let us agree that owning things makes us feel a little better. Not dramatically better, especially once the novelty has worn off in a day or week or month but a little bit better.
Second, owning things make me feel a little less afraid. As human beings we have the ability to sense things. We are not only aware of our selves and our surroundings but are aware of our awareness in a second-order thinking and feeling process that may be unique to us as far as we know. Of course what we know is not much and is based on the past. Past experience, past knowledge, passed down to us. Owning things makes us feel a little less vulnerable in this vast emptiness known to us as the universe, a little less afraid. Again we are just talking this over here and we might agree that we are nowhere close to being fearless. If we were we would not be bombing our neighbors or killing people we deem dangers to society. If we were really fearless we would simply let people be and treat everyone with dignity. We do not do that. Because we are afraid. Actually we are very afraid because we do not understand life and we certainly do not understand death. Buying things makes us feel just a little less scared.
So the reason for buying things, for the acquisitive instinct, wanting to own is a simple twofold one: By doing so we feel a little better and are a little less afraid.
We could learn to feel better and become fearless by finding truth within our selves. We bring attention to the dominant physical sensation in our body and follow that sensation until it leads us to a small truth. Then we follow the next sensation to the next small truth. When we do that for many many years we find that we do not need to own so much. We feel a little better and are less afraid.
We are also less weighed down by things and are thus free.