Why can't we just observe things like that spot of mustard you have left on your chin.

According to quantum physics you cannot just 'observe' something, for in observing 'something' you must alter its course. For to make an observation you must interact with the object you are observing.

The quantum model of nature encompasses principles that contradict not only our everyday experience but out intuitive concept of reality.

Nature does not dictate the outcome of any process. Outcomes are not determined with certainty, rather nature allows a number of different eventualities.

This bothered Einstein, but the truth/fact is that God may well throw the dice (for you theists) but he does not know the outcome.

Source:  Stephen Hawking & Leonard Mlodinow The Grand Design