In discussing the second of the Ten Plagues, the Midrash mentions that every creature plays a role in G-d's master plan. If not for the frog, says the Midrash, how would the Almighty take retribution against Pharaoh?
Why does the Midrash single out the frog when there were nine additional plagues? The second plague was key because both Pharaoh and the frog shared a common appearance of apathy.
The Apathetic Pharaoh
Pharaoh appeared apathetic in his denial of G-d. One can either accept the existence of G-d or reject it; but Pharaoh did neither--he simply ignored it. He regarded himself as the creator of his universe and never bothered to deny the true creator. He treated G-d as a non-entity, one that does not even merit denial.
The Apathetic Frog
In a sense one can argue that the frog too appears apathetic. Some creatures are clearly beneficial to the ecological system and enhance the lives of all creatures on the planet. Some creatures appear destructive, selfishly treating themselves to their every wish with no regard to the delicate balance of nature. Some creatures, such as the frog, neither contribute nor abuse, they simply are.
A Fitting Match
Both Pharaoh and the frog seem apathetic; Pharaoh to G-d, the frog to nature's cause. It was only fitting that G-d combat Pharaoh's apathy with a creature that itself appears apathetic.
Reflection
By using the frog G-d communicated another important message. The outward appearance of apathy is simply false. The frog's appearance of apathy disappeared as he responded with alacrity to the call of the Almighty. Though some appear outwardly so, deep down we all believe. We all have a mission to perform. We all play a role in G-d's overall scheme.
By Lazer Gurkow 
