At least four things necessary for solitude would seem to be: 1) experience, 2) discipline, 3) study, and 4) maturity. By these I mean several things.
First, by experience, that a person should learn not only what the world is like but what they themselves are like, monitoring the self, its feelings and responses. This guides a person to cultivate experiences which enhance not degrade. Second, discipline is the ability to discern where one’s desires and attachments stray and to control or channel them towards better things. There is physical discipline as well as mental discipline, and both should be pursued. Third, study can be cerebral book-learning (invaluable in today’s world) but also acquiring or developing a skill or sensitivity, what Gardner calls one of the “multiple intelligences.” It can be any of ten such; his books are useful discussions of what too many people assume is just one intelligence, that taught in schools: logical-linguistic. A skill (even a sensitivity or avocation), which is the directing of an intelligence, puts one in touch with the self and its place in the world. Lastly, a solitary should be mature and come to this state of life and mind consciously, not out of rejection, pity, or thwarted ambition. We have to “earn” solitude.