Reams of research has shown positive thinking to be good for your health. Optimists have better immune systems and tend to live longer and happier lives.
And as Winston Churchill said: "The optimist sees opportunity in every danger; the pessimist sees danger in every opportunity." Positive thinking puts you on the lookout for the possibilities in new situations, rather than just the potential pitfalls.
A positive attitude can be learned The techniques and attitudes of positive thinking can be absorbed and learnt at a deep level. When you instinctively start to see set backs as temporary and positive outcomes as inevitable then you'll have picked up the positivity habit.