An allergy is an overreaction of the body to a specific substance. People can for instance be allergic to nutrients (lactose), substances in the air (pollen) or cosmetics ingredients (perfume). An allergy can also build up gradually until the immune system develops hypersensitivity to particular compounds. Once someone has started getting an allergic reaction to a compound, that response will recur with every new contact.
The number of people who have allergic reactions to cosmetics is very small. However, no cosmetics manufacturer can guarantee that its products will cause no allergic reactions at all. After all, an allergic reaction is an individual response. People can for instance be allergic to strawberries, nuts, lactose, pollen or antibiotics. This does not mean that these compounds are harmful in general.
Stress may also play a part. Stress affects the resistance and makes someone more susceptible to an allergic reaction.