We all have to live life and we can not escape from it because it is a life cycle where one depends on another and vice-versa. It is quite simple, The Children depend on their parents and further the parents depend on their children. We all here to perform our duty and a part of universal life cycle. If one does not perform his duty or responsibility, there will a disturbance in the life cycle, he will get hatred, loathing and infamy. And if One performs his duty or responsibility very well then he will get reputation, honour, fame and glory and it also affects to his Reincarnation. So the luck of Reincarnation depends on the deeds of the previous birth or current birth too. And this is the base of 'GEETA'.
So In short the purpose of our lives is to perform our duties and responsibilities (Law of Karma). We are the part of life cycle and society and to continue it we all need to live our life.
If one does not perform his duty or responsibility But what if someone take sanyasa? that will also disturb the life cycle, but taking sanyasa is not also a bad thing right?
It depends on situations such as If a person is already married and he has one or more children who really rely on the person. In such case A person takes Monasticism (sanyasa) definitely, it will disturb the life cycle. In that case the sanyasa of that person is just like a Suicide. If anyone who wants to do like this then he or she should not get marry. A person should not run away from his/her responsibility.
A person is associated with so may responsibilities towards his parents, wife, children, brothers-sisters.. If A person has performed all his duties or taking sanyasa does not affect anyone in his family then it is worth to take sanyasa for that person.