Why should you avoid eating between the meals?

Plaque is formed, which in turn leads to tooth decay, which may then result in filings or having the tooth removed.

People can become overweight; they may lose their appetite at meal times, which means they don't eat enough healthy foods.
It increases caloric consumption (which not always is a bad thing) and it stimulates the bacteria growth in your mouth and on/in your teeth which can cause dental problems.