It is easy when we find ourselves at work, in class, or otherwise occupied with activities that are not what we consider fun to think of what we would rather be doing. This is one of the worst things that we can do.
For starters it is demoralizing. Rather than comforting us with pleasant memories it actually provides a contrast to what we are doing at the moment. This serves to make the good times seem much better, but they also serve to make the current toil seem that much harder. It is like the starving man being tortured by being forced to watch others eat a banquet.
Also it is a distraction from the work we are doing. If our minds are not where our hands are we will make mistakes. Even when we feel that we can do the job well enough that we do not need to think about it we will still only deliver the lowest level of competence we are capable of. With our mind on the work we can learn and improve our performance.
Last of all it is insulting to the moment. Every instant of our lives is unique; the toil and the play are equally valuable experiences. When we live in the future or the past we are throwing away our lives.
This is not to say the momentary wandering of our thoughts to what we are doing tonight or having for dinner is bad. What is bad is when we actively choose to put our mind somewhere else while we work. When we choose to be other then here in the moment it only serves to increase our dissatisfaction.