In addition to Vineet Menon's comments, that Agni (Fire) is the purest element of the 5 elements (Mahabhutas) that is accessible to man - (The other begin Ether (Nabha or Akasha) is intangible and is capable of sustenance and not incineration or destruction.), there is also another reason to it, in that, cremation is the best possible way to symbolically signify the transformation of the Gross (or Sthula Sharira attached to the Annamaya Kosha) to the Subtle (or Sukshma Sharira attached to the Pranamaya Kosha). This is also the reason camphor is lit for mangala aarathi in temples, where the camphor signifies the sthoola and the sublimation (disappearance) of the camphor when lit by fire (which is the 'Chit' or consciousness) signifies the sookshma.
And as rightly put by VineetVineet, realized souls and children devoid of ego (ahankara) are buried and not cremated.
By the way, the Holy Bible does say "From Ashes to Ashes and Dust to Dust". - Food for thought!