Ideally, I'd like to build everything, but realistically, the cost of equipment and materials is usually a deterrent. I'm also pretty lazy, but that's mostly when I'm not doing something I want to do (which, unfortunately, is most of the time). Also, I'm a perfectionist, so if I want it done masterfully, I may have to pay for it.
Ultimately, whether you build or buy, you invest and reap the benefits thereof. Building reaps the benefits of more tools, experience, cuts/scrapes (and scars if you're lucky), stories, and the end product. Buying reaps the benefits of less tools (yes, that can be a benefit if your garage is as cluttered as mine), fewer cuts, and usually a better finished product, which, oftentimes is more reliable than what you'd build yourself (unless you already had the experience and tools to build it).
Build if you can...buy if you can't.