The Hindus believe that hosts of invisible beings dwell side by side with them in their houses, taking their places over certain places and objects. This fact is emphasized by the observance of certain ceremonials like the performance of sraddha where these invisible entities are requested to go away from particular localities to enable them to perform their ceremonies in peace.
Some of these entities and forces are said to be in streets outside houses during the daytime. As soon as the night falls after sunset, they enter some one of the houses from the street to take rest during the night. They did it just as a benighted traveller would be filled with delight at the sight of a restaurant or hotel with lights burning and tables ready, laid out. These benighted entities and forces of nature welcome a house kept tidy with lights burning. They readily enter it and if they find the people and accommodation suitable for them, they stay there permanently, making the inmates happy and prosperous by the influence radiating from them.
All good entities of the invisible world around this living world are said to love light, and abhor darkness. All bad and evil denizens of that world execrate light and take delight in darkness. A bat and an owl love to roam in darkness, while the other birds like the parrot seek their nest after sunset. A bat and an owl are birds of ill repute, said to be serving witches while the parrot and other similar birds that delight in light are said to serve the devas like manmatha. In the same way a parrot is said to be as the brahmani. Kite, peacock, swan, etc., are said to be the vehicles of Vishnu, who is the preservative aspect in the universe. Lord subrahmanya, who is the second-born of Siva and Parvati representing wisdom, and Brahma is the creative aspect and so on.
With this strong belief in the background, people take great care to have lights burning in shut up houses to prevent entities of evil repute from entering them and making them their permanent abodes.
The goddess Lakshmi is said to abide in money, flowers, betel-leaf, saffron, turmeric, the burning flame of a light and so on. If they are given to others during the daytime, the link connecting the entities presiding over them to their owners is not snapped by the effect of the solar light. And because of this reason they return to their original abode before sunset, as if by instinct through this connecting link so to say. But if they were given after sunset the link is lost and the entities are in the position of children taken to other places while asleep. In course of time hardened to their new surroundings and people benefiting them and enriching them by their radiating influences. This appears to be one of the chief reasons for the reluctance of the Hindus to part with or lend certain objects to friends and others after the lights are lighted after sunset and nightfall.
In trying to understand the 'why' of the Hindu customs, one should bear in mind that they were originated by very great men among the forefathers of the Aryans and others. Hence, they ought not to be lightly disposed of. There is a popular dictum as 'There is much in the Hindu custom that we are not aware of.'