One old paradigm is that the brain works like a container. You only have enough space to fill it up so every piece of information that you remember counts towards your maximum capacity (think of your brain has a hard drive). Sherlock Holmes certainly subscribed to that theory which is why when Watson told him that the Earth revolved around the sun instead of vice versa, the former merely told him that he will try to forget it as it is unneeded information. Today, some people still subscribe to that belief but that's not really the case when it comes to the brain. Because of pattern recognition and the firing of neurons, the amount of information we can store seems infinite (and that is the real question--our brain might have a maximum capacity but no one really knows what that capacity is). That's not to say memorizing a lot of things doesn't cause us to "forget" others. In psychology class, I learned about interference and how one fact interferes with another fact. For example, we saw our Jill wear black on Monday and blue on Tuesday. On Wednesday, we forgot what Jill wore and expect it was either black or blue, based on our previous experiences with her (of course this assumes Jill has that much limited of a wardrobe). If we saw Jill wearing black on both Monday and Tuesday, we might have expected her to wear black on Wednesday too.
In line with that, there's what I first heard from Dean as "magic bullets". Every person has a finite number of magic bullets that enable them to come up with great ideas/concepts/stories/inventions/etc. (If you're fan of the Supernatural TV series, think of it as every person is armed with The Colt.) Once we exhaust those supply of magic bullets, everything we come up with is shit. The thing is, every person's supply of magic bullets is different from the other person (some are blessed more, others less) and more importantly, we don't know how many we've got. Assuming you subscribe to this theory, that means whenever you come up with a masterpiece, that masterpieces counts against the total masterpieces in your lifetime.
It's an interesting theory albeit one I personally consider as outdated as the brain-container model yet it resonates with a lot of people's fears. For example, when we come up with a great business idea or this one terrific story, we try our best to protect it and immediately cry fowl when someone tries to "steal" our ideas (i.e. "The Matrix was our idea"). I think in many ways, we cause such a furor because whenever we come up with such masterpieces, we feel that we've just won the Lotto and that the chances of producing another similar work is nil. (Of course the other reason why you should press charges is because of the principle, that somebody stole something from you, especially if you're confident you'll come up with more masterworks in the future... you can't just let everyone who passes by steal your ideas.) In college, it's much like coming up with a good topic for your thesis. There's certainly a lot of topics that can be covered with any given subject but we feel that there's only a few good topics that we can come up with and reasonably write about.
The theory also resonates with the idea of inequality. Following this theory, people like Da Vinci or Thomas Edison had lots of magic bullets. Other inventors, not so much. And while I think most people hope for a world where the playing field is equal (which is why people have a love-hate relationship with monopolies, love them if you're the one holding the monopoly, otherwise hate it the rest of the time), a part of them also recognizes that the world is unequal (and conclude that because of that, the world is unfair). This is where the hoarding mentality comes in and why some people buy ideas from other people as they would commodities. Personally, as a writer, it also strikes a chord in me. I wonder that with every story idea I come up with, that's one less concept I can use so I'd better make good use of it (of course the fact of the matter is, stories aren't just ideas--execution and actual writing makes the story a winner or not). And in many ways, that extends to blogging. Every blog entry I write today is one entry I won't be writing tomorrow. Thankfully I don't believe in the theory so I simply overwhelm you with entries (honestly, if the magic bullet theory was true for me, I'd have stopped blogging several weeks ago...two hundred entries in two months is too much for most people I think).
Third is that it resonates with our fear of death. With death, we don't know when it'll happen. Sometimes we have an inkling of the end but seldom a specific time and day (unless you hired the hit-man yourself). The same goes with magic bullets--not only do we not know how many we've got, we also don't know when it'll run out. We certainly don't want to wake up on the day that the well has finally dried, so to speak. It's also compounded by the death factor. One of people's so-called regret is not doing everything they could have done before they die. Assuming you're born with lots of magic bullets, isn't it a waste if your die prematurely before exhausting them? I expect on a subconscious level, some people fear that.
Of course at the end of the day, I don't believe in the magic bullet theory. In many ways, living such a life can limit us. Instead of giving 100% into your work every single day, you're tempted to hold back a little just so you don't exhaust your supply of magic bullets (or worse, spend your magic bullets for someone else's benefit). But I think what we should remember that as humans, we create, we invent--we make new magic bullets. That's why we're gifted with intelligence and creativity, so we can use them. Being a writer or an inventor isn't about any one product, it's about our ability to consistently create new material (whether it's a piece of fiction or an actual invention). We might never hit it big but that doesn't mean our contributions to society isn't any less meaningful. Go create your own new magic bullets. It's been my experience that the more you create, the more refined those future magic bullets become.