No Hindu scripture has given a comprehensive, systematic and a rational explanation for the four human objectives, called purushartha chatustay (पुरुषार्थचतुष्टय). However I see two conceivable reasons that could have prompted the sages to define purushartha chatustay — the four key objectives or goals that all devout Hindus ought to follow.
The first reason is discernible from the mantras themselves while the second reason, probably, lies in the four categories or varnas, that the Hindu society has been classified/divided into.
Let us take Manusmriti first. Chapter 2 mantra 224 of Manusmriti mentions only three purusharthas (objectives) and describes their virtues.
धर्मार्थावुच्यते श्रेयः कामार्थौ धर्म एव च ।
अर्थ एवैह वा श्रेयस्त्रिवर्ग इति तु स्थितिः ॥ २२४ ॥
Some deem Dharma, plus purpose in life, as “superior”; some deem desire and purpose in life as “superior” yet others prefer only Dharma as the summum bonum. The fact is that the triple combination of Dharma, purpose and desire collectively called “trivarga” stands supreme.
Manusmriti in chapter 4 mantra 176 says:
परित्यजेदर्थकामौ यौ स्यातां धर्मवर्जितौ ।
धर्मं चाप्यसुखोदर्कं लोकसंक्रुष्टं एव च ।
Leave all deeds and desires that are not in consonance with Dharma,
shun deeds and desires that are condemned by the society.
Mahabharata in Shanti Parva 164.4 (below) clearly states that Dharma, Artha and Kama all three should be savoured together. Anyone who engages himself with just one of them, does the abominable. The ones who engage with just two of them are deemed mediocre. Those who engage with all the three equally, are deemed excellent.
धर्मार्थकामा: सममेव सेव्या, यो ह्येकभक्तः स नरो जघन्य:।
दुयोस्तु दाक्ष्यं प्रवदन्ति मध्यं, स उत्तमो योऽभिरतस्त्रवर्गे ।।
Artha and Kama, I think, have been assigned an inalienable role in human life because without them — without a clear end in life and a firm desire to accomplish that objective or goal — the very human life would come to an end.
The desire to progenate, to produce offspring, to see them prosper and to help them prosper is the basic human instinct. No wonder then that the sacred Hindu scripture, Manusmriti, has devoted five chapters on the norms and rules that a devout Hindu must follow — particularly a Hindu who has settled in life, built up his household to follow the life of a right earnest householder, called a grahastha.
Garuda Purana Preta Khanda– Chapter 2-Shloka 4 as well as Devi Bhagavata Purana (4.15.16.) says:
अपुत्रस्य गतिर्नास्ति स्वर्गो नैव च नैव च ।
येन केनाप्युपायेन कार्यँ जन्म सुतस्य हि ।।
A person without a son has no place to go, neither here nor in the heavens.
Commenting upon a similar mantra in Valmiki’s Ramayana noted English Indologist Ralph T. H. Griffith says: “Sons and Paradise are intimately connected in Indian belief. A man desires, above every thing, to have a son to perpetuate his race, and to assist with sacrifices and funeral rites to make him worthy to obtain a lofty seat in heaven or to preserve that which he has already obtained”. (The Hindus believe that the souls of ancestors would never get salvation till their descendants offer the funeral rites and (तर्पण) tarpana. The eldest son of the family is the one authorised to perform funeral rites and tarpana. A person without a son has no salvation, say the scriptures.)
The two human objectives, namely, Artha, (the end in life) and Kama (the desire to accomplish one’s objective) always run the risk of going astray if they are not guided by a sound, ethical mind that is endowed with an equally sound code of ethical conduct. This role is done by Dharma which, like a lighthouse, guides a person all through his life. And this explains why Dharma has been placed as the first purushartha amongst the four of them.