It is the property of the aromatic organic compounds that they have sweet smell. there is no answer for "why". We use this property to distinguish them from other compounds.

An aroma compound, also known as odorant, aroma, fragrance or flavor, is a chemical compound that has a smell or odor. A chemical compound has a smell or odor when it is sufficiently volatile to be transported to the olfactory system in the upper part of the nose.

Generally molecules meeting this specification have molecular weights of <300. Flavors affect both the sense of taste and smell whereas fragrances affect only smell. Flavors tend to be naturally occurring, and fragrances tend to be synthetic.

Aroma compounds can be found in food, wine, spices, perfumes, fragrance oils, and essential oils. For example, many form biochemically during ripening of fruits and other crops. In wines, most form as byproducts of fermentation. Odorants can also be added to a dangerous odorless substance, like propane, natural gas, or hydrogen, as a warning. Also, many of the aroma compounds play a significant role in the production of flavorants, which are used in the food service industry to flavor, improve, and generally increase the appeal of their products.

It''s just how our brain respnds to their smell..It''s kind of like colour.