Unfortunately, the exact answer to your question still remains unknown. Some people have allergies because their immune system generates an unnecessary immune response to non-threatening proteins (e.g. those found in grass pollen). If the body identifies these non-threatening proteins as threatening, then an immune response is generated and the resulting symptoms are those related to the allergy. Unfortunately, why some people get allergies and others don’t is still largely unknown, however, genetics is likely to play a role.