Why do you put Tilak?

Tilak is  a traditional mark to identify devotees of Krishna. Tilak is made with sacred clay from a holy place in India, and it marks the body as a temple.
It is worn to remind the wearer, and everyone else, that within the body resides the individual soul and the Supreme Soul, Krishna.
It is a stamp of Lord Krishna’s lotus feet and His great devotee Tulasi on one’s body declaring that this body is the property of Krishna and should be used in His service only.