'Don't worry, be Happy' is an age old maxim, if one can put it that way. The act of being 'Happy' is referred to as the absence or dearth of worries in one's life and the presence of happiness, which may be due a wide variety of reasons. Everyone wants to be 'happy' and content in their respective lives. The definition of being 'happy' is rather relative and varies across societies and individuals.
Some people find it more difficult to stay happy than others often because of their pessimistic approach to life. No wonder, we see people striving too hard merely for being happy as they fear that happiness might just elude them. The act of being happy is more psychological than materialistic. 'How happy you are?' depends on how grateful you can be, at all times, even when the chips are down. Gratitude is the key to happiness, but the fact remains that nobody can be happy all the time.