Aromatherapy has been around and has been practiced in one form or another since the beginning of civilization. It is the art of using oils extracted from aromatic plants to enhance health and beauty. Apart from the physical benefits, essential oils can have subtle effects on the mind and emotions. The essential oils taken from plants and used in aromatherapy have been described as their "life force" - they are essential to the plants' biological process, as well as being the substance which gives them their scent. Synthetic oils, even if chemically similar, will lack the true elements and that vital life-force that make essential oils so valuable therapeutically.
Another reason why synthetic oils are not acceptable is that they are never identical. The addition of synthetic chemicals is not normally disclosed in the essential oil business, so unless there is a declaration that the oils are natural, pure and unadultered, assume otherwise.