What is the theory behind the doctrine of unjust enrichment?

The doctrine of unjust enrichment is based on the theory that individuals should be allowed to profit or enrich themselves inequitably at the expense of others.

The doctrine of unjust enrichment is based on the theory that individuals should not be allowed to ask for more in contract negotiations than the other party is financially able to give.

The doctrine of unjust enrichment is based on the theory that individuals should be allowed to freely and voluntarily enter into any contract they wish.

The doctrine of unjust enrichment is based on the theory that individuals should not be allowed to profit or enrich themselves inequitably at the expense of others.