The purpose of life is happiness. Does that mean accumulating the most toys? No – that would be considered temporary happiness. Haven’t you had the experience where you have achieved something, for example bought a new house, and then find a year down the line that you are restless and want something else. This would be an example of temporary happiness, and it comes from the outside in.
Unshakeable happiness comes when you cultivate the state of Buddhahood in your life. It is happiness which grows from the inside out. When you have established this kind of happiness, what happens in your external environment doesn’t matter. You will have that happiness deep inside no matter what happens in life.
When you chant Nam-myoho-renge-kyo you can transform all sufferings into stepping stones for establishing that state of Buddhahood in your life.