I now explain rationale or thought process behind two very important customs of India :-
1) Why we do not touch books or any other human being with our Feet
In Indian Custom touching book with feet is considered a sin , this is primarily because to Indians knowledge is sacred and divine and therefore it must be respected at all times . So not stepping on books or touching them with feet is a reminder of high position accorded to knowledge in Indian culture. It is believed that Goddess Saraswati resides in books and hence books should not be ever touched with feet. This is also the reason why we worship books once an year on Saraswati Pooja day , dedicated to the Goddess of Learning.
Similarly human being is regarded as the most beautiful , living and breathing temple of the Lord. Therefore touching another person with feet is disrespecting the divinity within that person.
Therefore even if by mistake someone touches book or any other person with feet one should ask for immediate apology ( not merely by lips but from Heart ).
2) Why Trees are regarded as Sacred – The life in us or plants or animals pervades all living beings and is considered as the Lord himself and hence they are considered sacred. Human life on earth depends on plants and trees. They give us food , oxygen , medicines and shelter. Hence as per Indian customs one is taught to regard tree and plants as sacred. We should therefore plant atleast 10 trees in our life time to compensate for one tree which we may need to cut for any reason. Certain plants and trees like Tulsi and Peepal are worshiped by Indians. It is believed that divine beings manifest as trees and plants and therefore it is worshipped and regarded sacred.