Thai cuisine is well known for its spiciness, with Som Tam (a spicy papaya salad) being a famous example. In fact, however, the secret to Thai food is a balance of five flavors: sour, sweet, salty, bitter, and spicy. Some Thai dishes have a careful blend of all these tantalizing tastes. Others are served with something to help deal with the overpowering spiciness. For example, Tom Yum Goong, which is sour and spicy, is often paired with an omelet or rice.
This could be the reason rice is always part of a Thai meal. As well as many herbs and spices used in Thai food, fish sauce is often used in a similar way salt is, as it mellows the taste. This means vegetarians will have to take this into account and be more careful when choosing food in Thailand.