1. Building a computer is like building legos, a very expensive lego that breaks if ur not careful :P
2. No, self-built computers are not always breaking down. Breaking down on computers depend on 3 things
1)quality
2)temperature of your system
3)how much stress you're putting on it
Quality means it'll have a much longer longevity vs an item with lower quality.
Temperature affects all. If you have high temperatures, your components won't cool off from all the work and may possibly melt. So that's why we like aftermarket coolers
Lastly, how much stress. Some people are crazy overclockers and would make the cpu work so amazingly hard, that it's like putting a 1000 pound man on a chair that could support only 300 pounds. Sure, the chair would support him for a while, but it will break down.