Pessimism is generally known as the attitude when an individual starts to think that he/she is not competent enough to control the situation at hand and then tends to slip into a passive stance where there is not much effort made towards the objective at hand. In short, the person gives up easily considering it all to be futile.
Pessimism is also a defense mechanism to reduce the effect of traumatic or disturbing events. There are those who think negatively so that if the outcome of an event is bad, they can console themselves by saying that they had anticipated the outcome earlier.
If everything goes normal, then it is assumed that the efforts that have been invested were fruitful. The major drawback of being pessimistic is that a person takes a passive stance and just allows the events to occur around him/her without trying to control or turn them to his/her advantage. The efforts that are put in are not sufficient and they are backed by the anticipation of failure.
Consider the example of a football team that is going to play against a rival team with the thought of losing the game. It is highly unlikely that they will win. However, there have been instances where sheer optimism has helped a weaker opponent win over a stronger one.
Pessimism is caused most of the time due to exposure to uncontrollable events which ingrain the thought of not being able to control a situation. This is where the second mental state called worrying starts. This is one of those states where the mind becomes agitated without one being involved in any event, but just by the knowledge of it occurring and anticipating a negative outcome.
Pessimism can be treated and a person must force himself/herself to think on a positive note that will spur actions and hence more avenues to gain control. This process will progressively pull a person out from such a state of mind.