[[Dear Sister, you write about eremitical solitude being a rare vocation. yet you also said that it is the most universal of vocations. So which is it? How is it that human beings can be such social animals and yet you can talk about solitude as a universal call. It sounds confused to me.]]
Three Main Forms of Solitude:
Thanks for your questions. The answer to your, "Which is it?" is not either/or but, as with so many things in Christianity, both/and. Part of what seems confusing is the use of the term solitude. It has a variety of meanings and these can especially differ if one is using it to speak of solitude in a world where the Christian God is real. Three main meanings in particular are important here. I therefore refer in a lot of posts here to physical solitude, existential solitude, and eremitical solitude. In the statement you are referring to I said that solitude itself was the most universal of vocations but it is the call to eremitical solitude which is very rare.
"Solitude" can first of all be used to speak of physical solitude, the state of being physically alone. I think this is often the meaning most folks attribute to the word. A hermit, who distances herself from so much sometimes called "the world" of people and events is certainly usually alone in this sense, but so are many others.
Secondly, "solitude" can be used to speak of the individual' s relationship to the world and its creator in the more existential sense; that is, it can point to the fact that we are each and all of us ultimately alone in this life and isolated from all others despite there being many people in our lives. Theologians speak of one aspect of solitude in this sense as the result of human sinfulness and therefore, as a result of estrangement or alienation from our deepest selves, from God, and so too from others. However, another, more positive side of it is our call to grow as individuals; especially in community we are not spared this call to individuation, this call to stand as integral and independent human beings. Still this existential solitude can be painful for it highlights both our most fundamental potential and deficiency.
Each of us knows this kind of solitude which is most intense when, for instance, we have acted wrongly, we are misunderstood, have been betrayed, feel alone or separate in a crowd, or simply have something too deep, or wonderful, or simply too difficult to share with anyone else; we know it especially when we consider death and the inevitability of dying alone. Even those we love profoundly and by whom we are are loved in the same way cannot entirely relieve us of either the challenge or the burden of this kind of solitude. In fact, the paradox of this kind of solitude --- whether as a call to individuation or as the burden of separateness --- is that it is often set in most vivid relief when we are with and are loved by others. In other words, this form of solitude is both most challenging and most painful because we are made for communion with others but are ultimately separated from them.
Hermits especially, embrace a life of physical solitude which sharpens our existential solitude so that we may live a contemplative life in the eremitical solitude of conscious communion with our God; the hermit knows this form of solitude as one which encompasses, but also transcends, and finally makes an ultimate sense of the first two senses of the term solitude. Because the hermit knows union with the God who grounds the existence of all creation, she also exists in communion with all those others in some incomplete or proleptic sense. When I have written about this before I have spoken of it as a solitude which redeems isolation and which provides hope to others that their own isolation can be transformed and transfigured, and so forth.
Communion is always implied by solitude and vice versa in human relatedness:
Vocations accent either side of the paradox of human solitude/communion:

In vocations to eremitical solitude the focus is different. It is on the solitary side of the equation. Most human beings are called to achieve human individuation and wholeness in communion with God through community with others. While hermits have already achieved an essential individuation before becoming (or even seeking to become) hermits (they could not embrace such a vocation otherwise) their growth in human wholeness and holiness occurs in eremitical solitude --- a solitude lived in communion with God for the sake of others. Very few human beings are called to achieve human wholeness and holiness in this way. Even so, they remind all persons of 1) their existential solitude, 2) the foundational communion with God which grounds and completes all human existence, 3) the place of community in even the most solitary of lives, and 4) the possibility of the redemption and reconciliation by and to God of even the most marked isolation or estrangement. At bottom then, this will always be a rare vocation and certainly always much rarer than vocations to marriage and community life.
While this answer may be longer than you expected, it is still quite a simplified presentation of the nature of solitude and especially of eremitical solitude. I hope you find it helpful in answering your question.