How do you see things when you are happy? And when you are sad?
Emotions are complex. They are related to your mood. You can be in a good mood or in a bad mood. Sometimes it looks like you feel well, but you may experience an affective change in your life which may imply some attitudinal changes. In that case, you try to control your emotional expressions in some ways but it can be quite difficult.
Negative emotions don't help at all. Everybody knows that. Experiencing negative emotions can lead to a physically unhealthy situation. However, positive emotions are more healthy. Optimism simply means expecting positive outcomes and it helps you feeling well and makes you face problems head-on when trying to solve them. It gives you some stability that pessimists don't have. Since you perceive things positively, your health is better. Be optimist, then! It's simply a style!!!