If you watch a newborn, you will see just how much effort they expend performing these two apparently simple tasks.
What I only realized recently is that in a broad sense we can be (and generally are, for most of our lives) spiritual newborns. Spiritually speaking our souls alternate between performing holy deeds like mitzwoth, hesed, prayer or learning, and removing the waste and damage done through less than spiritual endeavors, through teshuvah.
It's not really such a long jump to relate mitzwot (and everything else we categorized along with them) as 'eating' because we know that in Gan Eden, the souls are clothed in the mitzwoth they performed in their lifetime; just as the food we eat becomes the flesh our souls wear in this world. Along the same lines, the connection between the expulsion of waste from the body and the expulsion of waste from the soul (teshuvah) is apparent.
The really interesting question for all of us to think about is: Newborns grow up, and move beyond eating and excreting to many far more interesting goals; what comes next spiritually?
Yes, just as these two basic functions (eating and excreting) still play a real role in our day to day adult existence, our spiritual lives will forever revolve around mitzwoth and teshuvah, but the spaces in between is where all the real depth takes place, where relationships are built, and life is given meaning.
I believe that HaShem put all the deepest secrets of life (the universe and everything) right in front of us. If we really stop and think about life and living, growth and growing, we'll see just how far our souls can grow.