One of the first quotes I knew by heart was the one by Emerson which went, "Nothing great was ever achieved without enthusiasm." Was it for an essay or an article, I do not remember. This post almost wrote itself, as I substituted the word 'enthusiasm' for the world 'zeal', this being the last post in the A to Z Challenge.
It is as I grew up further that I heard the phrase "where angels fear to tread", which meant that while it was wonderful to be enthusiastic, it was folly to take that one step further and turn into an over-eager beaver, akin to a busybody.
"Never let your zeal outrun your charity. The former is but human, the latter is divine."
Thus, while being zealous is a virtue, there are occasions when one needs to keep it in check, lest it turn into fanaticism.