The convention of communicative understanding and expectation.
Words are tools to form a communicative understanding. When something has been orally or in written said through an agreement like "I see you tomorrow". The understanding is that "Ok, I see you tomorrow". With that creates expectation and a form of convention. When this agreement doesn't go through where the first person doesn't meet up as said will create some disappointment and a very small amount of mistrust.
Let's say the exact same scenario happens on a second time. With this may creates lower, same or higher disappointment, but the certainty of mistrust increases. If this starts to happen on regular basis or very often the belief we will have to the first person's words will falter and some cases, words might no longer find meaning..
We may oversee the importancy and what power lies within our words will inflict or influence on others. Therefore we shall pay higher attention on what we speak. With that will increase our control of communication and also lower the risks of disappointment and being mistrusted.