What causes fast and rapid blinking?

When a person gets anxious, their blinking rate usually increases. 

According to body language if a person is interested in someone, they might feel anxious around them and so excessive blinking might happen. 

Blepharospasm is a medical condition that causes excessive unwanted blinking.

When a person is put under pressure, they might start blinking more often than usual. An interview candidate might blink excessively when asked a difficult question.

Dry eyes or the lack of proper tear production might trigger excessive blinking. When a person blinks more, their eyes are moistened by tears.

If a person got afraid, their blinking rate might increase. (See What are the body language signs of fear?)

Excessive fatigue can result in excessive blinking.

If a person needs glasses or if they can't see well, the blinking rate might increase.

If a person has problems with their eyelids then they might blink excessively.

Tics are one of the common causes of blinking in children. As long as the tic exists, the child can blink at a faster rate.

If a person keeps blinking fast because they are always stressed, for example, then this person will learn the habit and their blinking rate might become fast by nature.

Bacterial infection can cause irritation in the eyes and result in excessive blinking.

Some people connect rapid blinking with lying. Those people assume that a person might get tense while talking because they are not saying the truth. (See Why do people lie?)

Some people connect the blinking rate with the speed of thinking. Those people claim that when the blinking rate becomes faster, it usually means that the person is thinking faster.