I was thinking this morning about how we wear a cover over our arm tefillin partly to prevent the friction with the inside of our clothes from rubbing the bayis and rounding the corners/rubbing off the black coloring, which would pasel the tefillin.
I then wondered why the skin of our arms themselves don't get worn away or discolored by their contact with our clothes all day every day (unlike the tefillin which are worn for only a short time).
The answer is clear. Our living skin is constantly shedding old skin cells and constantly giving birth to new skin cells. Because it is constantly "rejewvinating" itself, the skin that rubs against the inside of my shirt today is not the same skin that rubbed against it yesterday. Therefore the friction doesn't cause any wear and tear.
Dead skin (leather), on the other hand, does not shed old skin cells or grow new skin cells, so it gets worn out with contact with things of this world.
Similarly, the only way we can survive the friction and spiritual wear and tear of this world is by constantly rejuvenating ourselves by shedding the parts of us that need shedding and trying to grow in kedusha on a constant basis. Without constant hisgabrus b'chol yom, we will suffer the effects of contact with this world.
Be alive, don't be leather!
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