Happiness Not Given By Others :

Only those who are capable of being alone are capable of love. The capacity to be alone is the capacity to love. It may look paradoxical to you, but it is not. It is an existential truth…only those people who are capable of being alone are capable of love, of sharing, of going into the deepest core of the other person – without possessing the other, without becoming dependent on the other, without reducing the other to a thing and without becoming addicted to the other.

They allow the other absolute freedom, because they know that if the other leaves, they will be as happy as they are now. Their happiness cannot be taken by the other, because it is not given by the other.