We are all influenced by the emotional states of the people we interact with. In the presence of cheerful, sincerely kind, upbeat people, we find it easier to become more cheerful and positive. Conversely, in the presence of someone who is negative, kvetchy, irritable, and angry, we easily feel more negative or uncomfortable ourselves. The person whose state is more intense and enduring will have a greater influence on the states of others, for better or worse.
Because the state of enthusiasm is contagious, if you want to increase your own level of enthusiasm, it makes sense to be around other people who are enthusiastic. Even talking to an enthusiastic person for a few minutes is frequently sufficient to elevate our own state. At times just talking to an enthusiastic person on the telephone will raise our spirits.
Your own enthusiasm will have a positive influence on the lives of others. By being enthusiastic you will be doing many acts of kindness. At times, you won't even be aware that your enthusiastic state was helpful to someone else. This can add a dimension of motivation for developing a greater amount of enthusiasm. Not only will you gain yourself, you will also be helping numerous others.
(From "Enthusiasm: formulas, stories, and insights" (2002) p.82; available from http://www.artscroll.com)