Monks come in two kinds, the cenobites and the eremites or hermits. The cenobites live in community whereas the hermits go off on their own. Eremos in Greek means desert, and there are many different motives for moving into the desert either literally or figuratively. There are those whose serious psychological conditions make it impossible for them to function in modern society. Chris Knight is such a one, who, when asked about Thoreau, replied in one word, "dilettante." That's saying something inasmuch as Henry David was one monkish and solitary dude even when he wasn't hanging out at Walden Pond. Somewhere in his fascinating journal he writes, "I have no walks to throw away on company."
Others of a monkish bent are wholly sane, unlike Knight, so sane in fact that they perceive and reject the less-than-sane hustle of Big City life. Some are motivated religiously, some philosophically, and some share both motivations. I have always held that a sane religiosity has to be deeply philosophical and vice versa. I think most of the Desert Fathers would agree. Athens and Jerusalem need each other for complementation and mutual correction. Some of the monkish are members of monastic religious orders, some attach themselves as oblates to such orders, and some go it alone. Call the latter the Maverick Variation.
And of course there are degrees of withdrawal from society and its illusions. I have been called a recluse, but on most days I engage in a bit of socializing usually early in the morning in the weight room or at the pool or spa where a certain amount of banter & bullshit is de rigueur. I thereby satisfy my exiguous social needs for the rest of the day. Other mornings, sick of such idle talk and the corrrosive effect it can have on one's seriousness and spiritual focus, I head for the hills to traipse alone with my thoughts as company. But I am not as severe as old Henry David: I will share my walk with you and show you some trails if you are serious, fit, and don't talk too much.
I am a Myers-Briggs INTP introvert. Must one be an introvert to be a hermit? No. The most interesting hermit I know is an extrovert who in his younger days was a BMOC, excellent at sports, successful at 'the chase,' who ended up on Wall Street, became very wealthy, indulged his every appetite, but then had a series of profound religious experiences that inspired him to sell all he owned and follow Christ, first into a cenobium, then into a hermitage.
A tip of the hat and a Merry Christmas to Karl White of London for sending me to this Guardian piece which profiles some contemporary monkish specimens.