According to MSN Healthy Living, the reason we hiccup is still unknown. A hiccup is a sudden contraction or spasm of the diaphragm, and while it can be annoying, there is seldom anything to worry about.
When a person hiccups, the nerves inside the chest send signals to the muscles between the ribs and the diaphragm to spontaneously contract. A hiccup can be a response to a variety of triggers, including emotional stress, excitement, overeating, drinking carbonated beverages, swallowing air, fast changes in temperature, smoking and bingeing on alcoholic beverages. Hiccups that last for more than two months can indicate disease, but it is very rare occurrence.