I don't know why we put things off. But if I had to guess, I'd say it has a lot to do with fear. Fear of failure, fear of pain, fear of rejection… Sometimes the fear is just of making a decision, any decision, because what if you're wrong?
We have to learn our own lessons. We have to sweep today's possibilities under tomorrow's rug until we can't anymore - until we finally understand for ourselves what Benjamin Franklin meant: That knowing is better than wondering. That waking is better than sleeping. And that even the biggest failure, even the worst, intractable mistake beats the hell out of never trying.