David Hume, although he doesn't phrase it this way, argues that people become religious zealots because the ordinary duties of morality enjoined by plain-vanilla theism don't stimulate them enough. They want to show off to their deity by doing useless but painful things to themselves to overcome the boredom of a normal ethical life:

http://oll.libertyfund.org/?option=com_staticxt&staticfile=show.php%3Ftitle=340&chapter=44360&layout=html&Itemid=27

Perhaps the following account may be received as a true solution of the difficulty. The duties which a man performs as a friend or parent seem merely owing to his benefactor or children; nor can he be wanting to these duties without breaking through all the ties of nature and morality. A strong inclination may prompt him to the performance. A sentiment of order and moral beauty joins its force to these natural ties; and the whole man, if truly virtuous, is drawn to his duty without any effort or endeavour. Even with regard to the virtues which are more austere, and more founded on reflection, such as public spirit, filial duty, temperance, or integrity, the moral obligation, in our apprehension, removes all pretence to religious merit; and the virtuous conduct is deemed no more than what we owe to society and to ourselves. In all this a superstitious man finds nothing which he has properly performed for the sake of his deity, or which can peculiarly recommend him to the divine favor and protection. He considers not that the most genuine method of serving the divinity is by promoting the happiness of his creatures. He still looks out for some more immediate service of the supreme being, in order to allay those terrors with which he is haunted. And any practice recommended to him which either serves to no purpose in life, or offers the strongest violence to his natural inclinations, that practice he will the more readily embrace, on account of those very circumstances which should make him absolutely reject it. It seems the more purely religious because it proceeds from no mixture of any other motive or consideration. And if, for its sake, he sacrifices much of his ease and quiet, his claim of merit appears still to rise upon him in proportion to the zeal and devotion which he discovers. In restoring a loan or paying a debt his divinity is nowise beholden to him; because these acts of justice are what he was bound to perform, and what many would have performed were there no God in the universe. But if he fast a day, or give himself a sound whipping, this has a direct reference, in his opinion, to the service of God. No other motive could engage him to such austerities. By these distinguished marks of devotion he has now acquired the divine favor; and may expect, in recompense, protection and safety in this world and eternal happiness in the next.

So I have to wonder about the humanist analogs to this behavior to gain favor with some abstract Humanity in lieu of a god. Unlike traditional theist zealots, humanists generally don't signal their virtue through sexual self-denial, interestingly enough - in fact they tend to go to the opposite extreme and practically want to fit adolescent girls with Malthusian belts like Lenina Crowne in Brave New World, along with giving them the vaccine against cervical cancer. But some of them signal virtue through forms of consumption-denial, like veganism, voluntary simplicity,"low carbon footprints," buy-nothing observances and so forth. And then, after they've established their superior virtue in their own minds, they want to inflict it upon the rest of society by scolding us about our diets, our childrearing practices, our consumption habits and other aspects of our lives. This tendency bears more than a little resemblance to overtly religious behavior which makes little practical sense.