When a man life becomes complete

When you ask 'why', one can come up with an answer involving cause and effect looking at past. But if you ask what is the meaning of life. Or what is the purpose of life, I am guessing you are asking something else.

Life really has no meaning. The meaning and the purpose of life is a collectively imagined idea to convince that the race, the thing you try to do, the thing you try to get is worth the trouble. It is not. Life is to live at present moment, instead of sacrificing the present moment for some never arriving future.

One can argue the meaning/purpose of life is happiness. And that's why we do everything we do. Thinking that it will make us happy we end up in a roller-coaster ride of emotional highs and lows. If you argue in this line, only true lasting happiness, joy and fulfillment, is enlightenment. Then one can say that purpose/meaning of life is enlightenment. But this is still just an argument.

The question "why?" with regards to existence comes from theistic thinking. Even some scientists cannot free themselves from this kind of thinking. It is a result of believing that there's some ultimate entity/force in the universe that decides a purpose for us.

Buddhism doesn't subscribe to ideas like that. Buddhism teaches about causes and effect. It never talks about an inherent purpose or something written in the stars with regards to existence. Because there is none. But that doesn't mean that there are no goals worth aiming for. In an ultimate sense, end of suffering is the goal that is worth striving for.

If your head is on fire, trying to put off the fire is the smarter thing to do. Not to ask "why me?".

Your purpose and mission in life is the pursuit of happiness.

This includes avoiding suffering, and achieving happiness. Whatever that could make you happy, makes you complete. Even a newborn baby cries when she is hungry and is happy when she is drinking delicious milk.

Working on your hobbies, advancing your career, developing a family, nurturing a child, going on vacations, increasing your wealth, raising your status in the world, attaining fame in society etc. could all make you happy.

But the point is that these types of happiness would not last forever.

The goal in Buddhism is the end of suffering. But why do you want to reach the end of suffering? The reason is to attain the only permanent happiness, which is Nibbana.

So, the mission in Buddhism is really the pursuit of happiness, which is the purpose of your life. The Buddha discovered that both over-indulgence and over-asceticism are not conducive to the path to the end of suffering. So, he prescribed the middle way through the Noble Eightfold Path.

If you follow the middle way, you can have long term, medium term and short term goals of happiness.

The long term goal associated with this mission is attaining Nibbana (permanent happiness). It might take multiple births, maybe even aeons to achieve this.

The medium term goal would be to try to achieve: at least stream entry for Theravada and strong cultivation of Bodhicitta for Mahayana. You can also get more info on stream entry in this YouTube video talk.

The short term goal would be to try to achieve and maintain "materialistic" happiness in this life and future lives (which includes avoiding unfortunate rebirths). At the very minimum, you need to keep the five precepts with heedfulness (appamada). Going a little further, you need to practise more of virtue (sila) with heedfulness (appamada).

This short term goal is described in the Ittha Sutta:

Long life, beauty, status, honor, heaven, high birth: To those who delight in aspiring for these things in great measure, continuously, the wise praise heedfulness in making merit.

The wise person, heedful, acquires a two-fold welfare: welfare in this life & welfare in the next. By breaking through to his welfare he's called prudent, wise.

For lay followers, there is plenty of advice on achieving the short to medium term goals in the Gihi Sutta (or Discourse to the Householder), Sigalovada Sutta, Dighajanu Sutta and Anana Sutta. The minimum training rules imposed on lay followers are the five precepts. Also see this answer for the question "Can a Buddhist own and run a billion dollar business?"

For those in a hurry to Nibbana, there are the more advanced training paths of anagarika (sort of a pre-monk or pre-nun), novice monk or nun, and fully ordained monk or nun. Please see this answer for details.