Sometimes I have nothing to say
On some occasions there are many reasons, as to why there are no words available. It may be, that you are uncomfortable with yourself or the situation that you are currently in. While there are times in life when nothing needs to be said, there are many occasions when you will have little choice but to speak, especially when the conversation requires you to make decisions etc.
Some people are quiet by nature and seldom involve themselves in conversations but prefer to listen.