When I form an opinion of something, I tend to wonder whether the opinion was based solely on the matter at hand, or whether unrecognized external factors may be confounding my perspective.
Consider an example: I am currently re-designing a local company's website layout. I have long been intersted in web design, and I still am, yet I'm recently finding myself disinterested with the work I'm doing.
A simple analysis might go like this: I'm doing work, I'm not enjoying the work I'm doing, therefore I don't like the given profession. Translating that to my particular example would be like saying that web design is merely something I thought I would enjoy, but that I lack passion when it comes to doing it as a job.
Yet I think that there are other factors at play causing me to dislike what I'm doing. Perhaps there is something about the work environment that is frustrating me, perhaps I am not sleeping enough and the tiredness is putting me in a bad mood. It could even be some form of malnutrition throwing off my mood. The source could be so disconnected and unrelated to work that I could have no good guess at what it is.
From personal experience, my best guess is that it's from insufficient sleep.