The scent of flowers is mainly due to the presence of volatile organic compounds (VOCs) in their petals. These VOCs are aromatic molecules that are released into the air and picked up by our nose, creating an olfactory sensation.
Some plants are genetically programmed to produce more scent than others. Other flowers may not have a noticeable scent for a variety of reasons. This may be an evolutionary adaptation to discourage herbivores from eating them, or simply a characteristic of the species.