The inevitability of an early exit from a tournament (whether intentional, by accident, or from impatience of wanting to get tons of chips right away) will happen to you often.  This can usually be avoided with a sound foundation of basic strategy, along with a developed sense of patience and centeredness seen in ascetic Buddhist monks who don't eat much.  However, the human species and its environment is entirely imperfect.  We will often not get past the first few hands of $10/$20 blinds because of it.  It is of great concern to some, since poker appears to be a game of no excuses.  I find that attitude shallow and misanthropic.  Things happening to you and your local universe will cause your early exit. There will be nothing you can do about it, except to be happy with the fact that you are still alive playing poker online.  

Here are my reasons for my early exits:

1.  My home internet cable went out for hours just before I started this sit-and-go that I paid X amount of dollars to play in.  I am not going to haul myself to the local boba tea shop with wireless and play poker in public.
2.  I just find out that my mom was rushed to the hospital, and after winning about $10,000 worth of chips in some freeroll, I donk it off all-in and get rid of the stack, and run out the door to the emergency room.  
3.  I never lay down my top pair/top kicker, even though the huge overbets tell me otherwise.  
4.  I like going all-in with AA and KK and QQ and JJ and TT within the first few hands.  
5.  Beer.  
6.  After spending hours reading and rereading my strategy book, I forget everything I need to know about pot odds, semibluffing, value bets, position, hole card strength, and I'm out of the freeroll in one hand.  
7.  I don't know what I'm doing.
8.  Things just couldn't be helped.

Early exits are the bane of many poker players, getting the money in good in fast tourneys before getting blinded out, and beating yourself up because you should have been more of a nitty, tight player for over twenty rounds of blinds.  But when the earthquake hits, the power goes out, and the tsumami drags you and your computer into the ocean, you really need to get a reality check, and find a rowboat to dry land.