To answer the question, I have never felt called to the single state of life, because it is not something I or any of us are called to.
This comes up every few months here on CM, I urge everyone who is still single and thinks of this as a fall back position, "maybe I am called to be single, to please do the research, as there is no calling/vocation to being single. It is a state of life not a vocation (to which one could be called).
Tossing around the word vocation with the word single is a post Vatican II feel good sort of thing that has occurred to make single people feel less left out, like their lives are still important, or less like they live "on the Isle of Misfit Toys" (to use a "Rudolph the Rednosed Reindeer" analogy). So you will see even priests use the term "the single vocation", but it is not theologically correct.
There are three states of life (ordained, married, unmarried) which is a factual, demographic categorization, but two vocations (consecrated holy orders/avowed life and marriage). That is it.
Some people by nature are never going to be married (such as the "eunichs" referred to in the new testament), there are some people who are going to forgo the natural law "calling" to the married vocation and by that sacrifice do some good service (being a missionary in Africa, being the dutiful son or daughter serving an elderly parent in need--the urging that St. Paul made to us), but that does not make it a vocation. That was a decision or a service or a ministry best done by maintaining the single state of life, but it is not a vocation.
It does not make it bad or not of good value not to be a vocation, it just is not one of the two vocations.
You are either called to the consecrated life (priest, nun, consecrated virgin living in the world) or you are called to marriage.
A vocation requires three things: a call (from God), a fiat (the acceptance of the call, the yes from us), and this is the thing: a VOW to someone else (formal, public, lifelong, irrevocable). That is only holy orders/consecrated life, and marriage that have all three elements.
Just sort of deciding or by some inaction or confluence of circumstances remaining single does not involve any of that, and particularly does not include the last (the vow).
The vast majority of us are called to marriage. Those who are long not married need to first reconsider whether they were in fact called to the consecrated life and just missed it somehow, or if they are still called to the vocation of marriage, figure out why the fulfillment of it has not occurred (either due to self, to circumstances, selfishness, concupiscence or whatever). Unless you are consecrated virgin living in the world (and that requires a bishop and a ceremony and a vow, which is not most of us, if you are just watching hanging out still, working, trying to be a good person and citizen, and are single, that is great, but that is not a vocation.
Okay, end of lecture! This is just my pet peeve, and I worry how many wonderful called to the vocation of marriage people there are that lose lots of precious time and opportunity to marriage because of this point. It breaks my heart!