Being insincere means pretending to feel or think something that you don’t actually feel or think. This can be very tiring because you need to keep up the act, making sure you don’t give yourself away, no matter how you are actually feeling or what you are actually thinking. This means you expend extra energy and effort making sure your words and actions match the attitude or opinion you are pretending to have. In the end, it can be emotionally and physically draining.