You've probably heard (from a college professor or pretentious douche) of the Myth of Sisyphus. Sisyphus was forced to roll a boulder up a hill for eternity, only have it roll back down again before he reached the top. If this sounds more like your everyday life than a metaphor for the futility of existence, then it might be time to get a new career. Of course, jobs like manual labor or retail can often feel numbingly repetitive, but any job can become endless drudgery if you don't believe in the value of what you're doing. The best salesmen escape the repetition because they truly believe that they are matching each customer with the best product for them. The best laborers treasure the subtle differences in the ways they ply their trade each day. Even the most elite jobs in business or the arts can feel repetitive if you lose touch with the value of your work. If you don't believe in the work, it is only a matter of time until you can't motivate yourself to do it.
3. You Don't Believe in the Work