Why estimations are always wrong?

There are a lot of discussions around estimation in software development, about how to make it simple yet still effective to help the whole team including customer achieve the most. 
Bad estimates are one of the most persistent problems facing managers everywhere. Project and task estimates tend to be off. - always! Estimates are not created by machines. They are made by people with human foibles. Politics, pressure and optimism all come into play when it comes to estimating. Here's where they can go wrong and why.