Some suggest the purpose of human life is happiness, but no references to explicit statements in the scriptures were provided.
Are there explicit statements about the purpose of human life in the scriptures?
Yes, in the Mrigendra āgama the purpose of human life is mentioned.
atha dehādi sāpekṣaṁ tatpumartha prasādhakam|
tato niyati sāpekṣamastu karma niyāmakam||
So, if it is maintained that karma itself is capable of accomplishing all the four goals of human life - dharma, artha, kāmā and moksha – for the souls depending on body, instruments and others, then that karma is to be held as depending on niyati tattva also. So, it is niyati tattva that is responsible for the regulation and restraint of the karmic fruits. Chapter 10, verse 16,17.
Moreover first three goals are said to be inferior by the same āgama, these 3 are just to accomplish last goal because one can't attain moksha without exhausting previous Samskars.
mastrivargavāmatvāt rahasyañca svabhāvataḥ||
vāmaṁ dhāma paraṁ guhyaṁ yasyāsau vāmaguhyakaḥ| ,
Since the three goals – dharma, artha and kama – are of inferior nature when compared to the highest state of liberation(moksha), they are collectively known as ‘vama’. That effulgence(deva) of Siva which enables the souls experience dharma, artha and kama according to their karmic fruits becomes known as ‘vama deva’. Since, by its own nature, it remains very subtle and secret(guhya, not exposed to vision), the ‘Vamadeva’ mantra is considered to constitute the privy part of the form of Lord Siva (vamadeva guhya) Chapter 3, verse 12,13.
Purpose of human life in Chandogya Upanishad is said to be Moksha. -
Higher than the immense self is the unmanifest;
Higher than the unmanifest is the person;
Higher than the person there’s nothing at all.
That is the goal, that’s the highest state. CU. Valli 3, verse 10.
Lord Krishna seems to reiterate this in BG as well-
This unmanifested state is called the Indestructible. They call that the highest goal which, having obtained, they return not. That is My highest abode. BG.Ch 18, verse 21.
EDIT:- From this link, I have found a beautiful verse.
सोपानभूतं मोक्षस्य मानुष्यं प्राप्य दुर्लभम् | यस्तारयति नात्मानं तस्मात् पापतरोत्र कः || ततश्चाप्युत्तमं जन्म लब्ध्वा चेन्द्रियसौष्ठवम् | न वेत्तात्महितं यस्तु स भवेदात्मघातकः ||
Who is the greater sinner than the one who, having found an elusive human embodiment close to liberation, does not save himself? He who, having found this best birth, does not [apply] the efforts of the senses, does not comprehend the good of the soul, he is the murderer of the Atman. (कौलावलीनिर्णय, प्रथम उल्लासः / Kaulavalinirnya Tantra, the first chapter, Shloka 31-32)