There have already been many excellent theories presented here. Yet I'm still going to add my views, even if many of it's aspects can be found in other answers already and all those probably apply to some degree to this great scene.
What Tyrion might be ultimately explaining could be the cruel and senseless life in general, his treatment by the world and his father in particular, or even an attack on Orson Scott Card. But what he might also be talking about is nothing but the Game of Thrones itself.
All those various houses in Westeros do nothing else the whole day than crushing beetles, destroying men, women, enemies, allies, other houses, their own kin and their own lives for nothing but the seat on a damn uncomfortable chair that doesn't come with any actual achievement at all. Whoever sits on the Iron Throne today can be sure that tomorrow everyone else is keeping on crushing beetles to answer the question who gets the next turn on the Iron Throne. All this ultimately senseless Game of Thrones is bound to go on forever and ever and Tyrion, being one of the few people in the whole world with a bit of common sense, can try to understand what all this is for, but he won't, since there is nothing.
The only thing that will put an end to this whole Game of Beetles is an equally senseless and ununderstandable natural disaster from the outside, be it a mule kicking you in the chest or a horde of White Walkers covering your precious Iron Throne in endless winter (or a dragon army downright melting it away, for that matter). Yet cousin Orson was lucky enough to really die, Westeros will get a new chance and a new summer and be plagued by the Game of Thrones forever and ever, as it did before.
(And of course don't take those ramblings as any kind of spoiler, as I haven't read a single page of the source material, which you don't have to for recognizing such a society's circle of demise.)