art by ~lovelylittlesunshine
It is generally agreed upon and taught to everyone since we are little that we should develop the "good" feelings in us and work on minimizing the "bad" ones. Accentuate the positive, eliminate the negative. We are so used to this that we go through life taking it for granted. We genuinely try to be "better" people, to overcome fear and pain and be braver, to overcome irritation and be non-judgmental, to disregard tiredness and occasional apathy and push on, never quit, to overcome feeling depressed or disappointed and to always think positively. Nonetheless, we still experience the "bad" emotions and feel guilty for it.
But I believe that negative emotions are a natural part of us, and in moderation, they are necessary.
However nice it is to be positive and think positive, we will not be able to exist without negative emotions, because they signal something is wrong, make us aware of the surroundings and thus, protect us.
For example, fear is a form of our survival instinct. To have no healthy fear is not wise and can bring our demise. Pain lets us know when we are in a dangerous situation. People who have a very high pain threshold (for example, with severe Diabetes) can step on glass without noticing and end up with infected wounds, or have a heart attack without feeling the chest pain and not seek medical help. Tiredness signals we need to rest and replenish our energy so that we don't drop, and so on.
Negative emotions spur us on. When we do not like the present circumstances, it makes us work toward something better, something more acceptable, something more comfortable. If we are not satisfied, it may help us become better and more successful. Oftentimes, growth and progress occur not in spite of unpleasant experiences but because of them.
Negative emotions are evidence of our sound mental health. We may dream about the perfect life, but in the real world the good and the bad mixed in together. And we have to react adequately. That means to accentuate the positive and to notice the negative, and adjust our behavior accordingly.
The trick is to be balanced, to match the negative reaction to the scale of the adverse event. We should distinguish between real tragedies and nuisances in our lives.
When we go through a life altering experience such as a divorce, onset of a serious disease, loss of loved ones, etc., it is normal to go through the stages of grieve that include denial, anger, rage, envy, sadness, depression, regret, fear, detachment, and more.
If we get a traffic ticket or a bad grade in school, a lot of the above emotional responses would be an exaggeration that can throw us out of balance.
In his book How to Lose Control and Gain Emotional Freedom, Jerry D. Duvinsky, PhD writes that we are conditioned to think that emotions such as grief, anger, despair, helplessness, or loneliness are inherently bad, evil, dangerous, or wrong, so we feel the need to control them, suppress them, or disregard them. Granted, they are uncomfortable, powerful, and at times rather inconvenient. But our attempts to avoid them may produce deeper problems and lead to more suffering.
Negative emotions are intrinsic and indivisible part of us that helps us adjust, survive and improve ourselves. Instead of spending much effort to suppress them, we should recognize that unpleasant emotions are just symptoms of something else happening. Otherwise, we may give into them and behave in a destructive way. For example, quitting a job because of giving into feeling not appreciated by colleagues who did not invite you to an office party, or cheating on spouse because of giving into feeling angry with him for not putting the toilet seat up.
Instead, we should accept that life is not supposed to be perfect or easy. We will waste a lot of our vital energy wishing to never get hurt, scared, or disappointed. Rather, we should be glad that we have the ability to distinguish the good from the bad in our lives. We should acknowledge full range of our emotions as our faithful messengers of our environment, without labeling them "bad" or otherwise, and learn to recognize and deal with their cause, instead of focusing on the emotion itself. For example, it is not the pain that is our problem, but the nail we stepped on. We can suppress the pain by taking pills, but we would be much better off removing the nail. Instead of being overwhelmed by an emotion, we should resolve its cause. So if we get a bad grade in school, we should not mope around and feel stupid, but study to get a better one on the next test.
Negative emotions are a necessary part of us. So if we try to ignore them and smile despite anything, then firstly, our life can be endangered, secondly, we cannot react to circumstances adequately and wisely, and thirdly, we can develop personality disorders. If we ever will reach the mental state when we think only positive thoughts and smile all the time, it is possible we've gone crazy. So make room for some negative emotions in your head. And as always, remember that everything is good in moderation.
 The Kübler-Ross model (a.k.a., "the five stages of grief"), which hypothesizes that when a person is faced with a life altering or a life threatening event, he/she will experience a series of emotional "stages": denial; anger; bargaining; depression; and, acceptance.