Estimates become self fulfilling 

If we estimate that something is going to take two months to complete, then our mind is wired to believe it should take two months. And out of no malice, we begin to prioritize accordingly.

If we think two months is plenty of time then we let other things interrupt us. 

We also don’t feel compelled to work quickly. 

We’re more likely to do things that are possibly unnecessary simply because we have plenty of time. Might as well strive for perfection and not just success!

If we see two months as a short period of time, we likely rush ourselves and spend a lot more time worrying than actually doing. 

We have lots of meetings about what we’re going to do when we don’t get done on time. 

And the general consensus is that we won’t be done on time anyways, so why fret over doing the actual work.  We are going to have an extension anyways. 

By the way, largely, none of these actions that I’ve listed above are conscious decisions. They’re subconscious thought processes. 

Nobody is doing this out of intentional neglect. That’s the potentially scary part. It’s hard to be conscious of the impact a simple deadline can have.

At the end of the day, both optimistic and pessimistic outlooks result in two months or more to complete said goal.

Now for the icing on the cake. 

We all know that estimates are always underestimated.

When we pick a deadline and we don’t make it, the first excuse we turn to is that estimates are inherently inaccurate. It’s almost impossible to predict everything that will be necessary and that’s why estimates are always underestimated.
This further solidifies the fact that not only will it take two months but more

We have quite a few subconscious biases working against us. Now, I’m not saying never use estimates. Just that you should be cognizant of these when you decide to use estimates.