The capacity to be alone is the capacity to 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 persons 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.

Then why do they want to be together?
It is no longer a need; it is a luxury. 

Try to understand it.
Real persons love each other as a luxury; it is not a need.
They enjoy sharing: they have so much joy; they would like to pour it into somebody.
And they know how to play their life as a solo instrument.

The solo flute player knows how to enjoy his flute alone.
And if he comes and finds a tabla player or a solo tabla player, they will enjoy being together and creating a harmony between the flute and the tabla.
Both will enjoy it.
They will both pour their richness into each other.

Osho