It seems so simple, but it is often one of the most difficult tasks. Most people have a need for safety and security, and often this needs even precedes the need for stimulation and excitement. If you are known to break your word, people will feel uneasy around you. They will distrust you. They will dismiss your promises, and since it seems that you devalue them by not keeping your word, they will also view you with less value.
Humans are basically very selfish creatures.
While we might rejoice in someone else's good fortune, we also feel sorry for ourselves that the good fortune wasn't ours. Don't stir up jealousy and envy among friends by continually bragging. Let others see your good fortune, without you making a point of pointing it out. When you brag, you are basically trying to make yourself feel good at the expense of making someone else feel bad. The natural tendency to compete has no place in the arena of friendship.