Promise means being faithful to your word. No will will trust you if you break promises, Live upto one's promise. If you promise someone to do something, that means you are assuring them you will do whatever they have said you at any cost. If you are incapable to act upto your words then never ever make promises and break someone's heart.

Here's an heartbreaking story where you can see what are the consequences of breaking a genuine promise. Once upon a time on a cold night, a billionaire met an homeless old man who was shivering on street unable to withstand the severity of Coldness. However, he was standing still and had made up his mind to tolerate the cold until morning. The billionaire got down from his car and asked," Don't you feel cold being outside and not wearing any coat"? The old man replied, "I don't have any coat but I'm used to the cold. 

The billionaire asked the old man to wait and assured that he would bring the old man a costliest coat as well as blanket and would arrange an house to him within few days. The old man got very happy and said that he would wait for the billionaire to come back. The billionaire as soon as he entered the house he got an important call and got busy in his work, get completely forgot the poor old man. The next morning when the billionaire remembered the old man, he immediately went of out the house to search for the old man, he found the old man dead in the same place where he met that old man. The old man had left a note to the billionaire saying, "When I didn't have any warm clothes I had the power to fight the cold because, I was used to it. But when you assured and promised to help me, I got attached to your promise and that took my power of resistance to cold. The billionaire was filled with lots of guild and he started to cry on the middle of the road. He will regret this for his whole lifetime. 

Moral of the story: Never ever promise anything to anyone if you can keep the promise. One small promise can save life as well as take life. So make sure that you won't get into unnecessary promises and if you get into  a promise never ever break it.