You try to put in a good day's work every day, yet, often you find yourself unmotivated to do it. There could be a myriad of reasons for this - you are bored, undervalued, feel stuck in the wrong career, underpaid, unfair boss, office politics, bad work culture, etc. It is okay to feel bad or unmotivated for some time. It is unreasonable to expect yourself to be upbeat about everything all the time. But you must somehow learn to do good work despite all these reasons. Even in a job you don't like, and in a company that gives you ample reasons to hate it, you can still learn to do good work, be reliable, and cordial.

This is not the same as seeing the glass as half full. If reason tells you that you are in the wrong place or career, you have to take steps to get out of it. But while you are still there, every time you feel down, after the strong phase of that emotion passes, practice to pick and put yourself back on track to do good work day in and day out, no matter what. Don't while away time doing mediocre work.

You may subconsciously assume that doing bad work is justified and that your company or your boss doesn't deserve the best from you. This silent revenge is, first, morally wrong, and secondly, does more damage to you than to the company. You can't do bad work in a job for years, no matter how good the excuse is, and then suddenly be able to do great work when you get a job in an ideal company, or when you start your own business. By that time, doing bad work has already become your habit.

So, when is it justified to do bad work? Never.