The past does not, and cannot, specify the nature and position of every person in the world or event in the future. What it can do is describe the underlying fractal pattern which creates them.

"You are stealing me, you will steal me, you have stolen me. Oh my, tenses are difficult, aren't they?" - Idris

Future is important, as previously[1] mentioned. But so is the past. If we had a chance, just one chance to change a single thing in our past, what would we? Would we do something personal, change something in our past, or go for something more... global? Stop ourselves from making a past mistake or simply kill Hitler? If we chose either, we still have to think of the consequences. Every action worth changing causes a ripple, a line of events that follow. If there was no Hitler, perhaps there would've been some strategic genius instead? If we had not made a mistake in the past, perhaps we would have messed up even greater a bit later?
Once we have decided on a change and carried it out, we can observe the consequences. We, and only we, know the original timeline. So if the current situation turned for the worse, would we mind? It would be natural to mind making a mistake on a global scale, and especially so if the change directly affects us personally. Then again, if nobody else know how bad we messed up, why should we?
Regret is a natural reaction. Certain events trigger certain actions that are unlike us, extremely improvised and not of our nature. Even if nobody else knows, we know. We know that we did something we wouldn't in our right minds. We know we did something risky without a safety net. It's purely based on emotions and not logic, after all, regret is rather useless as one cannot change the regrettable action. Well, unless we enter the current hypothetical situation, in which we are painfully likely to cause even more regret.
Changing the past is a decision that has to be made with a heavy heart, ready to cause great pain. Even if we don't create a paradox, we could create something even worse.