One of the biggest mistakes, I think, that "happiness searchers" make is that we think we should be happy 100% of the time. Not only is this an unattainable goal, but being happy 100% of the time would probably be unhealthy as well. (I am also reminded of a statement I quoted a few months ago on this blog, which went something like this: "If you were happy 100% of the time, how would you actually know you're happy?")
Over the weekend, while reading the Czech popular magazine called Reflex, I ran across a quote by a noted psychotherapist, who said that a psychologically healthy person should be sad 10% of waking hours, upset 10% of waking hours, and happy and satisfied 80% of waking hours. I am not sure to what extent this is supported by research, or how other psychologists would react to this, but I think the idea that we should actually be sad or upset twenty percent of the time is a pretty good one.
Of course, this split should be taken more metaphorically, rather than aiming to actually have 10/10/80 every day. But I really do find it to be a good guideline, one that can help us be more fully human, which, at the end of the day, is where the search for happiness should take us.