Well there are many theories, Stories and angles for this question’s answer. I will try to give all the possible answers here.
I will take references from the Buddhist Bardo Thodol and the Hindu Garuda Purana to answer this question. The jeeva (spirit) exits out of the body at the time of death and for 11 days, it remains as a Pretha, after which it would proceed to the abode of Yama for his final judgement. A pretha is basically a ghost. Like humans, ghosts experience all kinds of emotions like anger, lust, and hunger but they do not have a physical body or a container to satiate those emotions or to let them out. During these 11 days, it is said that the ghost would be extremely attached to its previous body and family. Especially during the first three days, the ghost of the human remains in a state of confusion failing to understand its existence outside the body, which lies inert and lifeless. Due to the physical attachment to the body, they say, it constantly tries to get back into the body. This is the reason why Hindus insist to burn the dead body before three days.
Fire is considered holy in Hinduism. It burns away everything until nothing remains. On the other hand, burying is a very slow process of dissolving the five elements inside the body back into the five elements of the cosmos. By cremating the body, the physical remnants of the ghost are entirely wiped out from the face of earth, so that the ghost may continue with its journey forward after the 11 days. This also reduces the possibility of remaining as a ghost on the physical plane, for an extended period of time.
Garuda Purana mentions that people who experience untimely and unnatural deaths (due to accidents, suicides, etc) and bodies that do not get cremated as per the rites, remain as ghosts for a long time. This is because the physical body is considered a container of spirit and as long as it remains on earth, the essence and energy of the individual’s life still remains over. This is also the reason why in Hinduism, the bodies of great yogis, saints and sages are never burnt but instead buried and on top of it, they install a Shiva linga or make it a place of worship. The body of the sage or saint was a container of divine spirit and by burying it we let the divine energy or essence of the yogi’s physical existence, influence the people around it, positively.
Another story From Wiki.Answers
Hindus believe in the soul being indestructible; and that death symbolises end of the existence of a person’s physical being, but the start of a new journey for the soul. This soul then reincarnates in some other life form, and passes through the same cycle of taking birth, growing and eventually meeting death- only to begin the cycle afresh.
Cremation of a person’s dead body is therefore, supposed to rid the departed soul of any attachments to the body it previously resided in.
Also, a traditional belief among Hindus says that a person’s body is composed of 5 elements- earth, fire, water, air and sky. The cremation ceremonies of Hindus are directed towards returning the body to these elements. The body is progressively returned to earth, air, sky and fire by burning it under skies; and the ashes are respectfully collected and poured in a river.
It is said that excessive mourning over a deceased prevents the soul from being completely detached from its loved ones, and keeps it from undertaking its new journey- that of taking up a new life. Cremation (and subsequent ceremonies in mourning) help to remove most of those things that can act as a reminder for the person’s existence, and thereby also assist the family in getting over the loss.
This might be a scientific approach to the question :
A Human Being does not always die from old-age, he may die due to diseases. If he is burnt, the micro-organisms in his body will die (no pathogen survives at the temperature of fire). Thus, burning of a body after a person is dead preventing it from being a source of spread of any disease.
Also, isn’t it better to burn a body rather than let it rot naturally? Hindus also do not believe in burying the body because frankly, every tomb occupies space.
Not everyone in Hinduism is cremated. Very young children are not cremated, rather buried because they do not have an ego. They don’t even understand attachment to life yet.
1st Story: Vamsi Emani
2nd Story : Wiki.Answers