Our sages tell us (Avos 5:18):
ה,יח [כ] יהודה בן תימא אומר, הוי עז כנמר, וקל כנשר, ורץ כצבי, וגיבור כארי--לעשות רצון אביך שבשמיים. .
Yehuda ben Teima says, be brazen as a leopard, light as an eagle, swift as a hind, and strong as a lion to to the Will of your Father in Heaven.
One may ask why the Yehuda ben Teima didn't just tell us the midos necessary for avodas HaShem: one needs (sometimes) to be brazen, light, swift, and strong to do HaShem's Will. There are really two parts to that question: First, what additional information is being conveyed by giving us examples from the animal kingdom that exemplify each behaviour. Secondly, why do we need to approach avodas HaShem that way?
The sefer Achas Sh'alti brings an answer to both questions from a R' Tzedaka. By using animal examples, Yehuda ben Teima is telling us that we must incorporate these traits into our very being. We are not striving to be as strong as a lion, rather we are striving make strength so much a part us us that we don't need special exertion to exercise it. A lion has a mane and a lion is strong; those are just a part of him. A Jew needs to become so identified with being brazen (at times), light, swift, and strong that they are a natural part of him; like eyes and ears.
To the second point -- why? If a person needs to exert himself to be swift, then sometimes it works and sometimes it doesn't. That is not a proper approach to avodas HaShem. We have to realize that avodas HaShem and doing the Will of our Father in Heaven is not "extra credit", it's "de rigueur" -- it is simply the way a Jew lives in this world.