The equations describing a field theory contain terms which are the product the field, the thing it couples to (could be another field) and the strength of the

The equations describing a field theory contain terms which are the product the field, the thing it couples to (could be another field) and the strength of the interaction. When a field couples to itself, there are terms non-linear in the field. So the non-linearity of GR indicates that the gravitational field couples to itself, i.e. that gravity gravitates.