When it comes to spring and summer and all the pollen that seems to be in the air, most people want to know a bit about allergies, and they specifically ask the question "Why do allergies make you sneeze?" These maladies, it should be remembered, are due to an exaggerated response or overreaction by the body's immune system when the body comes into contact with some substance that can cause such a response.
The list of things that can cause an allergic reaction in people is essentially endless. As pointed out in the precedent paragraph, pollen is a common culprit in warmer months. Cat dander – which is very fine and can enter the nasal passageways quite easily – is another common foe. These two substances, in fact, are the most common causes of sneezing and watery eyes, which are classic allergic reactions in many people.
Currently, estimates are that around 50 million people in the United States suffer from some sort of allergy or another. Sneezing is quite common in a form of the malady known as "allergic rhinitis." The more well-known name for this condition, by the way, is "hay fever," and there probably is not anyone around who has not thought that he or she has suffered from it in the past.
Due to those pollens, which are easily inhaled (just like animal dander indoors), the tissues that line the interior of the nose (called "mucus membranes" or mucus lining) can become inflamed. Remember, this is a reaction to the substance (pollen or dander, for the most part) by the immune system. Sometimes, the ears, the sinuses and the throat can suffer just as badly from the inhaled substance.
Sneezing is a reaction to the insult (the intervention of dusts or pollens or dander) and a way in which the body will try to expel those substances. That's why you might see mucous production, some coughing or post-nasal drip which creates an urge to clear your throat and other autonomous responses which, group together, are called an "allergic reaction."