Every time you eat the Ph in your mouth or the acidity rises. When acidity is high and you brush your teeth straight away  you actually remove more enamel from your teeth. That can make them sensitive and wear off faster. If you do not brush your teeth you have the same effect. If you skip a brushing or two you might get away but after 48 hours your teeth are completely covered by plaque and the Ph becomes acidic, similar to having a meal or two.  Enamel becomes softer and more vulnerable to wear and damage. So that is why it is important to brush twice a day for minimum two minutes.