"Can" -- to be able to; to have the ability, power, or skill to
"Will" -- am/is/are about or going to
To say that we "can" or "cannot" do a thing is to say whether such a thing lies within our grasp, within our ability. It is a mere statement of fact, an observation of reality.
To say that we "will" or "will not" do a thing is to state a determination to use or not use our abilities. It is the articulation of choice.
"Can" implies acceptance of and submission to the reality of our lives. It underscores the limits of our power and ability.
"Will" requires determination and choice regardless of the reality of our lives. It is the affirmation of our power and ability.
Two small words, yet how we use them says much about who we are and how we live our lives. Those who merely "can" bow to the forces around them in abject submission; those who "will" receive the submission of those same forces.
