Well, for one, it's a lot better than anything you can buy off a shelf. You can decide what you want in it and not only build it for less than a store price, but every component is exactly what you need or even simply want. There's something stopping you though... Why is that? You know that it will be better, so why not do it?
I took a private class when I was 12 on how to build a computer. I honestly didn't learn a single thing except that I shouldn't be afraid of it. Everything I was taught was extremely simple, no difficulty on any level, and that was 13 years ago, now it isn't even half as hard.
Why should you build your own computer?
1. It will be better than anything on a shelf.
2. You won't be afraid to work on your own PC anymore, you'll be able to fix things yourself, saving time and money.
3. Your computer will be cheaper with better performance, especially if you're wanting a gaming system.
I will make a thread about how to build your own if anyone's interested. ^_^