The main theories are that life was created by a creator or natural random events. Life is a continuous process that once stopped, will not re-start. It can only come from previous life. In this case I only consider human life. Many of the concepts apply to other organisms as well.
Life seems to be it's own reason for being.
The reason for life seems to be to perpetuate life. The purpose of life would seem than be to enjoy life, be happy ourselves. To make other people happy. To bring joy. To make the worlds a better place for our passing through it.