Purpose of life

According to the Guru Nanak, the founder of Sikhi, the purpose of life or the goal of this human life is to become an "enlightened being" and to "enlighten others". In the beginning of Japji, Guru Nanak refers to God as "sach" meaning "True" - True at the beginning of time, ....:
Then in the first stanza of Japji the Guru talks about the purpose of human life in question and answer format:
Harmony with Hukam means concern for the ecosystem; treating all beings with kindness and respect for the environment. It is knowledge of Truth that destroys ignorance, falsehood, superstition and doubt. When the yogis asked Guru Nanak:
ਤੇਰਾ ਕਵਣੁ ਗੁਰੂ ਜਿਸ ਕਾ ਤੂ ਚੇਲਾ॥ … 
ਸਬਦੁ ਗੁਰੂ ਸੁਰਤਿ ਧੁਨਿ ਚੇਲਾ ॥ 
“Who is your Guru or whose disciple are you?”
“Shabad is the Guru and my mind, which is focused on the Shabad and comprehends it, is the disciple,” replied Guru Nanak. AGGS, M 1, pp. 942-943.
Shabad in the Guru Granth Sahib means God's Bani or teachings and Knowledge. So it is the understanding of God or knowledge of Truth that is needed for the understanding of Hukam (Cosmic Law). The one, who understands the Hukam and conducts himself/herself, accordingly, is a sachiara
(gurmukh, sadh, sadhu, sant, bhagat, and braham gyani). All these words are used interchangeably in the holy Granth. 
According to Guru Nanak there are two types of people, Gurmukhs (God-centred beings) and Manmukhs (self-centred beings). A Gurmukh dwells on God's attributes constantly and does every thing according to Hukam whereas a Manmukh is a degenerate person who does every thing according to his/her own will. A Gurmukh sees God everywhere and in each living being whereas a Manmukh is blind to God. Gurmukh follows Hukam whereas Manmukh follows the dictates of Haumai. 
The Guru Granth Sahib asserts again and again that one should not become slave to Haumai and the five passions/drives - lust, anger, greed, attachment and egotistical pride; rather one should subdue Haumai and make the five passions subservient by exercising restraint, so that one's energy and talent are directed to positive thinking and creative activities. One who achieves this objective conquers the corrupting influence of Maya (worldly temptations). 
Haumai is a deep malady but hope is not lost for a Manmukh since God is gracious, loving and forgiving. The door to Truth is always open. Naam Simrana/Naam Japna (constant focus on God's excellences) inculcates humility, love and compassion that lead to truthful living. This process gives one the inner strength to control one's passions and Haumai. Slowly and steadily one makes progress to subdue/restrain Haumai and the five passions, thus becoming a Gurmukh:
ਗੁਰਮੁਖਿ ਵੈਰ ਵਿਰੋਧ ਗਵਾਵੈ ॥ 
ਗੁਰਮੁਖਿ ਸਗਲੀ ਗਣਤ ਮਿਟਾਵੈ ॥ 
Gurmukh is free from enmity and hostility. Gurmukh
gets rid of ill will completely. (SGGS, M 1, p. 942.)
==More quotes from Gurbani==
ਗੁਰਮੁਖਿ ਨਾਮੁ ਦਾਨੁ ਇਸਨਾਨੁ ॥ 
ਗੁਰਮੁਖਿ ਲਾਗੈ ਸਹਿਜ ਧਿਆਨੁ ॥ 
ਗੁਰਮੁਖਿ ਪਾਵੈ ਦਰਗਹ ਮਾਨੁ ॥ 
ਗੁਰਮੁਖਿ ਭਉ ਭੰਜਨੁ ਪਰਧਾਨੁ ॥ 
ਗੁਰਮੁਖਿ ਕਰਣੀ ਕਾਰ ਕਰਾਏ ॥ 
ਨਾਨਕ ਗੁਰਮੁਖਿ ਮੇਲਿ ਮਿਲਾਏ ॥ 
Gurmukh reflects on God’s attributes, practices charity and keeps clean physically and mentally.
Gurmukh’s mind is focused on God/Truth constantly.
Gurmukh is honored in the realm of Truth. Gurmukh is exalted and fearless. Gurmukh does good deeds and inspires others to do so. Gurmukh units others with God, says Nanak. (SGGS, M 1, p. 942.)
ਸਾਚ ਵਖਰ ਕੇ ਹਮ ਵਣਜਾਰੇ ॥ 
ਨਾਨਕ ਗੁਰਮੁਖਿ ਉਤਰਸਿ ਪਾਰੇ ॥ 
We the Gurmukhs (God-centered beings, moral enlightened-beings) are the merchants of Truth. 
O Nanak, this way the Gurmukh crosses the ocean of corrupting influences of worldly temptations. (SGGS, M 1, p. 939.) 
ਮਨਮੁਖਿ ਭੂਲੈ ਜਮ ਕੀ ਕਾਣਿ ॥ 
ਪਰ ਘਰੁ ਜੋਹੈ ਹਾਣੇ ਹਾਣੁ ॥ 
ਮਨਮੁਖਿ ਭਰਮਿ ਭਵੈ ਬੇਬਾਣਿ ॥ 
ਵੇਮਾਰਗਿ ਮੂਸੈ ਮੰਤ੍ਰਿ ਮਸਾਣਿ ॥ 
ਸਬਦੁ ਨ ਚੀਨੈ ਲਵੈ ਕੁਬਾਣਿ ॥ 
ਨਾਨਕ ਸਾਚਿ ਰਤੇ ਸੁਖੁ ਜਾਣਿ ॥ 
Manmukh (self-centered being) is deluded under the corrupting influence of worldly temptations. He/she peeps into other’s home with a covetous eye and degrades himself/herself more and more. Manmukh is lost in the jungle of doubts/superstitions. The wrong path deceives him/her as he chants the mantras of spiritual death. Instead of reflecting on the Sabad/Truth, he/she shouts obscenities. O Nanak, it is the love of God/Truth that brings peace and happiness. (SGGS, M 1, p. 941.)
ਮਮਮੁਖੁ ਅੰਧਾ ਅੰਧੁ ਕਮਾਏ॥ 
ਬਹੁ ਸੰਕਟ ਜੋਨੀ ਭਰਮਾਏ ॥ 
ਜਮ ਕਾ ਜੇਵੜਾ ਕਦੇ ਨ ਕਾਟੈ ਅੰਤੇ ਬਹੁ ਦੁਖੁ ਪਾਇਆ ॥ 
The Manmukh blinded by Haumai commits evil deeds and suffers moral death wandering in delusion. Manmukh is wasted away in the end because it cannot liberate itself from delusion/ignorance and falsehood. (SGGS, M 3, p. 1068.)
ਪੰਚ ਦੂਤ ਮੁਹਹਿ ਸੰਸਾਰਾ॥ 
ਮਨਮੁਖ ਅੰਧੇ ਸੁਧਿ ਨ ਸਾਰਾ ॥ 
The whole world is deceived/swindled by the “five drives/passions -- Kaam (lust, sexual drive), Kroadh (anger), Loabh (covetousness, economic drive), Moh (attachment) and Ahankar (egotistical pride),” but the ignorant self-centered person [Manmukh] does not understand this fact. (SGGS, M 3, p. 113.)
ਕਾਮੁ ਕ੍ਰੋਧੁ ਦੁਇ ਕਰਹੁ ਬਸੋਲੇ ਗੋਡਹੁ ਧਰਤੀ ਭਾਈ॥ 
ਜਿਉ ਗੋਡਹੁ ਤਿਉ ਤੁਮੁ ਸੁਖ ਪਾਵਹੁ ਕਿਰਤੁ ਨ ਮੇਟਿਆ ਜਾਈ॥ 
O brother restrain your sexual drive and anger and use them as creative instruments like a farmer uses a hoe to remove the weeds from the field. You would be happy by following this course otherwise you would suffer the consequences of your bad actions. (SGGS, M 1, p. 1171.)
ਆਈ ਪੰਥੀ ਸਗਲ ਜਮਾਤੀ ਮਨਿ ਜੀਤੈ ਜਗੁ ਜੀਤੁ॥ 
O yogi! Make universal brotherhood your Aee Panth (highest order of yogis) and subdue your mind (Haumai) to conquer the worldly temptations. (SGGS, Jap, p. 6.)
ਪੰਚ ਚੋਰ ਆਗੈ ਭਗੇ ਜਬ ਸਾਧ ਸੰਗੇਤ॥ 
The five thieves: Kaam (lust, sexual drive), Kroadh (anger), Loabh (covetousness, economic drive), Moh (attachment) and Ahankar (egotistical pride) are subdued in the company of saints (moral enlightened beings). (SGGS, M 5, p. 810.)
ਨਾਨਕ ਹੁਕਮੈ ਜੇ ਬੁਝੈ ਤ ਹਉਮੈ ਕਹੈ ਨ ਕੋਇ॥ 
O Nanak, if one comprehends the Hukam then one does not speak with egotistical pride (does not act under the influence of Haumai (self-centeredness). (SGGS, Jap 1, p. 1.)
Finally, “physical death” is the fate of all living beings as it is subject to Hukam (Cosmic Law) whereas “moral death” (moral degaradtion) is avoidable and that is the point that is emphasized in the AGGS again and again. Gurmukh remains unaffected by moral death whereas a Manmukh experiences it all his/her life. We should be afraid of moral death and not of physical death, and we must strive to overcome moral death.
ਜੰਮਣੁ ਮਰਣਾ ਹੁਕਮੁ ਹੈ ਭਾਣੈ ਆਵੈ ਜਾਇ ॥ 
Hukam causes birth and death (creation and destruction) or birth and death occur according to
Hukam. (SGGS, M 1, p. 472.)
ਚਿੰਤਾ ਤਾ ਕੀ ਕੀਜੀਐ ਜੋ ਅਨਹੋਨੀ ਹੋਇ ॥ 
ਇਹੁ ਮਾਰਗੁ ਸੰਸਾਰ ਕੋ ਨਾਨਕ ਥਿਰੁ ਨਹੀ ਕੋਇ ॥ 
ਜੋ ਉਪਜਿਓ ਸੋ ਬਿਨਸਿ ਹੈ ਪਰੋ ਆਜੁ ਕੈ ਕਾਲਿ ॥ 
ਨਾਨਕ ਹਰਿ ਗੁਨ ਗਾਇ ਲੇ ਛਾਡਿ ਸਗਲ ਜੰਜਾਲ ॥ 
Worry about something that is unnatural. O Nanak this is the rule in the world that nothing is everlasting. Whatever takes birth dies today or tomorrow? O Nanak, shun all worldly temptations with corrupting influences and contemplate on God. (SGGS, M 9, p. 1429.)
Thus the purpose of life is to become a gurmukh (God centered being, moral enlightened-being) and to enlighten others.
* Based on article Nanakian Philosophy (Gurmat) The Path of Enlightenment