A person can be considered to be "light-hearted" if they are someone who does not dwell on problems, does not have a negative outlook on life. A light-hearted person would tend to think positively, have hope or faith that things are not as bad as they might appear. They "look on the bright side of life".

It means relaxed and pleasant or cheerful. Stress is bad for the heart. Therefore if you are lighthearted and cheerful you will live longer than someone who is always angry or depressed.