An Eager Beaver

Someone said to be "an eager beaver" is someone very excited and enthusiastic about doing a particular task.

Example: "Ron's been such an eager beaver since joining the company. He volunteers for everything and never complains."

While this is usually a positive trait, excessive enthusiasm can be annoying, or threatening, to some, especially in a very competitive environment.

A person can be an eager beaver for a particular task, that the person particularly likes.