The phrase "only too happy" is a commonly used expression that means someone is very willing or eager to do something. The word "only" emphasizes that the person is very keen on doing something with great enthusiasm. Some other synonyms to describe someone's eagerness could be "delighted," "ecstatic," "enthusiastic," "overjoyed," "pleased," "thrilled," "eager," "excited," or "jubilant." Depending on the context, one of these synonyms could be more appropriate and convey a different subtle meaning. Regardless of which synonym is chosen, each word expresses a positive emotion indicating a willingness to help or participate eagerly in a particular activity.