So a bicycle is a mechanical object.  anything mechanical can break.  it is just a fact of life.  it happens.  but from a number of years of riding and working at a bike shop and seeing problems with bikes i do notice things.  I can say most of your problems with stock bikes are the use of unsealed bearings in the hubs and cranks.  It is a weaker design and they tend to not take as much of a beating as a sealed bearing would. also when kids think they know what there doing but dont have the right tools to install things is generally when problems occur.  Ive heard of alot of troubles of frames breaking and stuff like that and honestly i can say alot of it might not be how hard your riding it.  it could be actual manufacturing defect.  weld area around major stress joints such as headtube and seat tube gets too hot causing the metal to get too brital and thus snap.  however, how hard you ride your bike does matter as well.  you cant have a single wall rim on the back of your bike and case a double and ride away just perfect.  doesnt work that way.  you also cant bail off your bike in mid flight and expect your bike to come away unharmed.  generally the first thing to hit the ground is your cranks, front wheel/fork area, and rear wheel.  which i have seen every one of these items discussed in this forum.  so pretty much to sum it up.  If your hard on shit then maybe you should beef it up a bit. if your real real smooth then a 20 pound bike is perfect for you.  ride whatever.  just know anything mechanical can break.