Since the beginning of the early man, even when they were oblivious to the act of worship or any presence of God, they still adored the natural elements in some form or the other. Even if they couldn’t acquire detailed knowledge about Almighty yet they expressed their gratitude towards the various elements of nature; as water, fire, wind, sun and soil were providing them with something. So the basic idea behind performing this act is to reciprocate the supreme love which is being showered upon us and expressing our gratitude for the selfless Universe.
We might say that even if we don’t perform the religious acts, we are still happy in our lives; so for thus Lord Krishna has said in the holy Bhagwad Gita;
“A-brahma bhuvanal lokah punar avartino rjuna
mam upetya tu kaunteya punar janma na vidyate”
Meaning- From the highest planet in the material world down to the lowest, all are places of misery wherein repeated birth and death take place. But one, who attains to my abode, O son of Kunti, never takes birth again.
So the few fortunate ones who realize the true nature of God, because of whom we exist in this world and survive, we perform this pious act to appreciate our existence and His divine grace upon us. Mortals have the biggest tendency of making mistakes, intentionally or unintentionally; so by remembering the divine power, honouring his divine presence and seeking forgiveness, we get free from the burden of guilt and rejoice our existence.
By being grateful for whatever we have received from this cosmic creation and feeling a sense of belongingness towards them, we express our honour that further leads to faith and devotion. Puja gives us the strength to surrender ourselves in the feet of holy power and staying constant at every situation. Thus, puja is an expression of the expanded consciousness honouring the whole creation and seeking for His grace.