From time to time most of us will accept something as true or as certain to happen, even though we do not have absolute proof (it is possible definitive evidence of the truth does not exist.

Making assumptions can be expedient but it could also result in inappropriate behaviour or doing the wrong activity because things we not actually as you assumed them to be.

Speculation

A close cousin to the making of assumptions occurs when we form a theory or conjecture without firm evidence, i.e., speculate.  Combining speculation with assumptions can lead to a very dangerous combination.

Basically, inadequate quality of facts and inconclusive information leads us to actions that appear to be supported by feelings or beliefs that we can firmly trust, although in fact they are based a bit on smoke and mirrors, (i.e., what appears to be, not necessarily what is).

Easy and Simple

The prime motivation for acting on speculative information and assumptions is simplicity.  It is easier to assume you know the answer and proceed than step back, dig in and investigate.

We can achieve our goals by knowing how to proceed and assumptions provide us with that direction.  Sometimes it works and of course, our level of false confidence drives our ability to make assumptions comfortably.

The thinking goes “We are good at this, it’s unlikely we are wrong.  Why wait, why do more work?  Let’s get it done.”  Unfortunately, this approach does not always work out.  Worse when it misfires you can really make an ass of yourself (or worse).