It's a fact that nocks break. 

But there's two ways of looking at it. Either complain about the cost of those little bits of plastic or congratulate yourself on your tight grouping! 

Personally, I like to look on the bright side. Each time I break a nock I congratulate myself on my superior shooting ability. If someone else breaks my nock, well, then i complain about the price of nocks... 

However you choose to look at it, owning a dozen spares is the sensible thing to do.