If you have read the archives about this, you know how to prevent new ones. In the meantime, you need to understand how these form.
Sebaceous glands make a fatty kind of secretion that helps keep our skin moist and pliable.
Since it is kind of thick, normally, is is pretty easy to clog the duct. When people hava a relative essential oil deficiency, that secretion tends to be thicker and more sticky. THEN, if one is not especially careful to keep any dried secretion from clogging the pore at the surface, the duct can be blocked. The more pressure there is in the gland, the harder it will be to get it open since pressure invaginates the lining of the gland into the duct. This is why surgical removal is the only real solution to a gland big enough to be a bother.