One of the staples of primary school writing assignments is the compare and contrast paper. The student take two things and has to writ how hey are alike and how they are different. The complexity changes as the children grow up.
Judging if today is a good day is also an exercise in comparing and contrasting. Deciding what you are going to compare and contrast with may be critical to how good your day is going to be.
If you want to be unhappy today, compare your life to the best life you can possible have. Don't compare it to the best thing you've ever had, compare it to the best possible.
Compare your breakfast, and your job, and your vacation, and your bed, and whatever you else have to the best of those things you have ever seen or imagined. Your life will fall short and you'll be unhappy. Congratulations.
On the other hand, while not focusing on "worst first" thinking, contrast your life to what it might have been had you made other choices.
If you are dreading your work on Monday, imagine your life without a job. If you are tired and don't want to make dinner, imagine not having a choice of food or food at all. If your family is driving you crazy, imagine your life without them. Maybe now is a good time to watch or re-watch "It's a Wonderful Life".
So next time you feel that your life is not what it should be, don't compare it to that magical world where everything is perfect, contrast it to what it might be and be grateful for what you have. And, for bonus karma, do something kind for someone who does not have everything you have.