There is no explanation, no justification for the solitary life, since it is without a law. To be a contemplative is therefore to be an outlaw. As was Christ. As was Paul.
One who is not "alone" has not discovered his identity. He seems to be alone, perhaps, for he experiences himself as an "individual". But because he is willingly enclosed and limited by the laws and illusions of collective existence, he has no identity. He is not yet conscious. He is alien to his own truth. He has senses, but he cannot use them.
To have an identity, he has to be awake and aware. But to be awake, he has to accept his vulnerability and death. Not for their ownsake: not out of stoicism or despair--only for the sake of the invulnerable inner reality which we cannot recognize (which we can only be) but to which we awaken only when we see the unreality of our vulnerable shell.
The discovery of this inner self is an act and affirmation of solitude.
Now if we take our vulnerable shell to be our true identity, if we think our mask is our true face, we will protect it with fabrications even at the cost of violating our own truth. This seems to be the collective endeavor of society: the more busily men dedicate themselves to it, the more certainly it becomes a collective illusion, until in the end we have the enormous, obsessive, uncontrollable dynamic of fabrications designed to protect mere fictitious identities--"selves", that is to say, regarded as objects.
In Christ, God takes to himself the solitude and dereliction of man: every man. From the moment Christ went out into the desert to be tempted, the lonliness, the temptation and the hunger of everyman became the loneliness, the temptation, and the hunger of Christ.
But in return, the gift of truth with which Christ dispelled the three kinds of illusion offered him in temptation (security, reputation, and power) can become our own truth as well, if we only accept it.
"Go out into the desert, having with you nothing of the world, and the Holy Spirit will go with you. See the freedom with which Jesus has gone forth, and go forth like him--see where he has left the rule of men. I will not make you such rich men as have need of many things, but I will make you true rich men who have need of nothing. Since it is not he who has many possessions that is rich, but he has no needs".