Here is a Rilke quote on solitude from Letters to a Young Poet:
"Only one thing is necessary: solitude. To withdraw into oneself and not to meet anyone for hours - that is what we must arrive at. To be alone as a child is alone when grownups come and go."
And here is Merton echoing the same theme:
"I am not defending a phony "hermit-mystique,' but some of us have to be alone to be ourselves. Call it privacy if you like. But we have thinking to do and work to do which demands a certain silence and aloneness. We need time to do our job of meditation and creation." (Contemplation in a World of Actions, p. 218)