Hindus, Buddhists, Jains as religions talk about being detached from the good and bad. They teach about going beyond attachment to the 'mine' in a strong way.
Hence, when the person dies, they burn the body clearly showing that the body you were so attached to (as brother, father, sister, etc) is not here anymore. It was mere karmic arrangement to have that person in your life and now it's over. Once burnt there is no hope of the person coming in that body and helps develop detachment, aiding the practice of letting go.
They also believe that this detachment helps the soul of deceased to move on faster to it's next birth as it helps him to let go of his attachments, too.