Overcoming Obstacles and Suffering with Tefillin

ל"ה - הלכות תפילין ה:יג

(Painting by Rivka Nehorai)

The reason for binding the tefillin shel yad on the left arm is because the left arm is the weak arm - the source of all suffering and obstacles.  The purpose of the tefillin is to counter these obstacles with patience and thereby enable a person to overcome everything.