Complaining is one way of feeling alive. Eating the bread God gives is a way of being alive. The first leads eventually to depression, a kind of death. The second is food for the journey and leads eventually, indeed anticipates, eternal life.
It seems that complaining is par for the course when it comes to human beings. We are just like our ancestors in that. One generation of ancestors in particular, those believers whose complaining resulted in losing the promised land. Complaining leads also to us losing the savour of the eternal gifts that are promised and perhaps losing the gifts themselves.
Why such terrible losses as a result of complaining? Because if we persevere in it we lose the capacity to receive what is given to us for what it is, a gift. The original sin means laying hold on the gift as if it is ours by right and so not a gift at all. It means ignoring the Giver to focus on the gift, another way of destroying its character as a gift. Then we receive life with, at best, a bad grace, and, at worst, with no grace at all.
So our ancestors complained and laid hold on the bread they had been given. But, says Jesus, they are dead. It is a salutary warning. The insidious power of consumerism messes with our desires and turns all our relationships into commercial ones. We are not sure any longer of the difference between what we want and what we need. It remains a present challenge, to appreciate the gifts of God for what they are. Even the natural gifts of trees and rain, tigers and weather, bread and wine.
The alternative to the complaining that makes us grasping and depressed and that leads to death is faith: the believer has eternal life, Jesus teaches us. The believer, open to receive, is drawn by the Father to Christ. Being drawn is a very different kind of capacity to grasping but who is to say that it is not actually a great deal stronger - indeed, infinitely so. People made able to be drawn in this way shall all be taught by God, they will hear (another kind of receptiveness) and will learn from the Father.
One way of receiving today's readings then (one among many) is that they give a teaching about desire, and how to approach life as a gift. It is all there, for us: how are we to receive it? By (imagine) imitating God (Ephesians 5:2), by following Christ who is the bread of life (therefore by eating him, the living bread). It means living eucharistically. And more will be unfolded about this as we continue to read through John 6.