Mountain Biking - Upgrading a new bike..what first?

I have read a lot about people getting new bikes and immediately taking off stock things and getting new, better things instead. For example, I was reading a post about someone's new bike in which they stated there was "Nothing stock except the frame left on the bike". I was wondering, what do people upgrade first and why? All I have done are pedals, grips, and seat..all for obvious reasons. I just don't understand why you would spend over 1k on a bike then spend even more on upgrades rather than building your own bike or getting a more expensive one to start. What is the first thing upgraded on a new bike, and why?

Any input?

Well, I'm at the point of having the frame, fork, seatpost and stem as the only stock things on my bike. First thing I really upgraded were the wheels, but that was after about 700 miles and I had bent the rear rim. I never really upgrade anything out of the box unless it doesn't work right, it just doesn't make sense, though if I was to do it I would probably upgrade the wheels or fork.

I have read a lot about people getting new bikes and immediately taking off stock things and getting new, better things instead. For example, I was reading a post about someone's new bike in which they stated there was "Nothing stock except the frame left on the bike". I was wondering, what do people upgrade first and why? All I have done are pedals, grips, and seat..all for obvious reasons. I just don't understand why you would spend over 1k on a bike then spend even more on upgrades rather than building your own bike or getting a more expensive one to start. What is the first thing upgraded on a new bike, and why?
Any input?

In my case, not counting pedals, wheels, because I tacoed the front.

whatever you break first. Don't fix it if it is not broken.

i would say tires if they don't fit what you ride.

I cut 0.75" off each side of the handlebars to make it shorter. Does that count?