A、Because they want to show respect to Buddha (佛). B、Because they think it will dirty their souls. C、Because they believe a soul

Thailand has some special customs and habits.
    Men end women generally do not touch or show affection in public. However, good friends of the same sex sometimes hold hands. Among the younger generation, it is becoming more common for members of the opposite sex to hold hands.
    In social visits, how one sits, walks, or interacts with others depends on the status of each person present. It is customary to remove one’s shoes when entering a Buddhist temple or private home. Visitors should avoid stepping on the doorstep because Thais believe that a soul resides in the doorstep of a temple. It is not necessary to take gifts when visiting, but it is not uncommon for guests on extended stays to present their hosts with a gift of appreciation.
    In the home, people commonly sit on the floor, but do not stretch their feet out in front of them. Women generally put their legs to the side and behind them, and men sit cross-legged. Men might also put their legs to the side to show special respect to the hosts. Guests may offer compliments on the home or children, but should avoid excessive admiration of any specific object to make the host embarrassed.