Your reputation is based on the things that you say and the things that you do. Granted, there might be other people who say negative things about you, but, in time, the truth always comes out.
Sometimes people hurt their reputation because they promise to do certain things and then do not follow through. Perhaps they offer to lend a book, prepare a handout, send a referral, or make a telephone call on behalf of the client. When they do this, the client usually smiles and is happy that you have made the offer. They depend on you to do this for them, but, if you forget or just don't bother, it won't take long until you have not only lost their trust but also have scarred your reputation.
You see people talk. They talk about the daily events in their life. They tell others what worked for them and what didn't work. They make recommendations about products and services. Word of mouth is an extremely powerful advertisement. So, if you have disappointed someone, the chances are that they will not be keeping this a secret. They will be telling their friends and family and neighbours.
I once heard that when someone is planning a funeral, they can expect approximately two hundred people to show up. Each person has a group of that size who care enough to go to the funeral. Well, if that is the case, just think about how a broken promise can influence dozens of people. The person who was involved tells several people who each tell several people. Soon there is a very large group who know that you broke the promise. They will likely not want to trust you based on what they have heard - because they trust the person who told them.
So, as a result, your business is hurt. You might never, ever know who or how many people have heard about your broken promise but the revenue and profit in your business will likely reflect this. So, the next time that you find yourself making a promise, write yourself a note immediately so that you will remember. Put the note in a place that you will be reminded and fulfill the promise that day. Do not leave the office until you have done what you promised to do. Then you will never have to think about it again - well, at least until the people brings it up again in the same sentence with the words "thank you".