Giving secularism, materialism, hypereroticism, and heterodoxy their ample due, the fact remains that I've known many admirable Catholic laymen who either have or do exclude themselves from consideration for the priesthood because they consider themselves unworthy. In the abstract, to be sure, they would acknowledge that nobody is worthy, that a genuine "call" comes only with a power to answer and live it that no man possesses naturally. But that truth of pastoral theology is basically seen as irrelevant. For it is generally assumed by practicing cradle Catholics that a man has to be better than most of us, and better in a certain hard-to-define way, in order to be considered "worthy" in the sense meant by Eastern-Christian tradition when the newly ordained are hailed as Axios!
Being "better" in the relevant sense does not mean being somehow more pious, or knowing Catholic teaching better, or even believing it more fervently, than the average Catholic man. I've known some laymen who are both more "pious" and theologically better-formed even than some priests I've known, but who would neither consider themselves nor be considered suitable for the priesthood. Nor is the problem any presumptive lack of willingness on the part of such men to live a life of ascesis and loving service. In the concrete, the sacrament of marriage as the Church understands it often requires living such a life to a greater degree than some priests do, or are even expected to do. I had dinner at a Jesuit residence last weekend and was amazed by how commodious their domestic life is; if this is poverty, I thought, then I for one wouldn't miss wealth. (A couple of the Jesuits there were guys who had known me in high school, and I thought the same about their lives back then!) I've also been inside parish rectories of which much the same could be said. True, most celibate priests are overworked and put under a microscope; but that's only true when they're not at home. When I was married with children, what I did on family time was often harder work than my job and certainly put me under closer scrutiny. Hence an old Irish joke: "Why is marriage a sacrament? Because nobody can crucify themselves."
I think the blockage here comes from a sense that a priest, unlike a layman, should be a transparent icon of Christ. When one meets a genuinely holy priest, his natural virtues are unprepossessing even if they happen to be great. They are not what one keeps one's attention when one meets him. Supernaturalized by the grace of his vocation, the natural virtues of a holy priest point beyond himself to Christ. It becomes evident that it is not the man who lives, but Christ who lives in him. It's not that a layman can't be that; but truth be told, laymen aren't at all expected to be that; largely for that reason, they are not seen to be that save in very unusual situations. Hence and perforce, they are not that.
That is why, when I was younger, hardly anybody saw me as "worthy" of the priesthood and eventually convinced me that I wasn't worthy. Since what they noticed was my natural self, not Christ, they assumed I wouldn't be interested in the priesthood; those who found that I was, assumed that my motives were natural not supernatural and thus unworthy. So, e.g., being theologically better-educated and more orthodox than some priests only earned me the plaudit "smart," and we all know that "smart" guys need to get themselves into lucrative professions. Being more traditional and consistent in my piety than some priests earned me only the shopworn adjective "devout," but that didn't even put me up there with the ethnic grandmothers, who were presumed to be more devout than anybody except the priests, who were presumed to be the most devout of all, even when it took rather little insight to see that they weren't. And of course, nobody with a noticeable sexuality counted as priest or nun material; if they were seriously willing to consider taking a vow of celibacy, that meant they needed a hot date and a psychotherapist rather than the seminary or novitiate. And I think it safe to say that my sexuality was, at least, noticeable.
It took me a long time to realize that that is how I was seen and, by and large, continue to be seen by most good Catholics who have known me in "real" life. Once I did realize it, I quickly noticed that many good Catholic laymen do the same number on themselves without any explicit social encouragement. They have seen only their "natural" virtues and conclude that the best way for them to serve God is to put those virtues to use in pretty much the way other good men do, only with the addition of quietly sanctifying them through the practice of their Faith. Thus, it is thought, one can become holy not only without believing oneself to be holy but also without appearing to be and certainly without talking about it. Such seems to be the picture—again, among most "good" Catholics—of how the laity, i.e. 99% of Catholics, are to live their faith. Those in consecrated life, on the other hand, are supposed to be icons. Unlike the rest of us Catholics, they can and ought to get away with being visibly holy and spending a lot of time talking about as well as living the Faith—even if their theology and piety are virtually indistinguishable from those of Katherine Jefferts Schori. But laymen who carry on with the God-stuff are vaguely (and often not so vaguely) suspect, even if their theology and piety are virtually indistinguishable from the Pope's. Such men are presumed to be eccentrics, or worse. It doesn't help matters that some of them are.
It is this bit of (for want of a better turn of phrase) Catholic collective psychology that helps to explain today's vocation shortage. It is taken for granted that only a rather narrow range of personality-types, which have never been thick on the ground, are plausible candidates for the priesthood. I don't believe that was always the assumption. Perhaps the difference now is that the priesthood, for various extrinsic reasons, isn't the path to social status and respect that it once was, so that Catholic men who could or do succeed in a secular profession are now expected to do so. Whether the change is a good or a bad thing is hard for me to say. But it definitely means that many good Catholic men won't join a club that might have them.