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It is simply not healthy, mentally and phyiscally, to be positive 100% of the time and even worse, to try to be.

If you accept that one "secret" to being healthy physically is to eat in moderation than it should be equally accepted that being healthy mentally or emotionally is also a moderation balance.

A better goal than that of being 100% positive is in learning how to deal with your negatives; how to turn those experiences into something of value.  You might treat learning curves as one step back for every two steps forward so in order to be evolving you must be experiencing your share of negatives, dealing with them and moving forward.

This is not to suggest that you should intentionally stick your finger into an open flame just to be a better person; to have something to learn from.  It is not suggested that you should look for problems only that a normal and expected act of nature is to be challenged and the test is in how you deal with it. 

To a certain extent, if you are ready to deal with challenges in a positive way you are being positive - therefore positive all the time but the reality is that many, if not most valuable challenges or hurdles do cause pain to some degree and positive value points do not happen until after you have dealt with the issue.

Keep in mind (back to moderation) that maybe the goal is to keep your ups and downs, your positive and negatives to a reasonable balance and attempting to be positive all the time is in fact being off-balance.