Why do Hindus cremate the dead?
Whoever is born must die. Amongst Hindus it is customary to do the last rites by consigning the body to fire. On this occasion, all relatives, friends and acquaintances get together to mentally convey a farewell to the departed soul. Their presence on this occasion reminds everyone of the ultimate truth - everyone has to die some day. It also reminds them of the futility of living only for oneself.
In the Chudaman Upanishad it is said that Brahma gave birth only to the flame-like soul. From the soul, the sky was born. From the sky, air was born, from air fire, from fire, water and from water the earth was born. These five elements united to form the human body. When a dead body is cremated in fire, the elements return to nature from where they came initially.
In the Atharva-Veda, 18/2/56, the cremation of a dead body is explained thus:
O Departed Soul, your lifeless body is offered so that the two fires may unite for your salvation. I set the body on fire. Through these two fires you may go in your best state to Yama, who controls death.