A child is
born.The first thing he or she does is to cry, the cry expands the lungs and then on it
breathes and does its karma.
Krishna is saying that
life is synonymous with karma. As long as you live, you continue working (do karma).
There is no way to escape. Humanity has freedom, but not as
far as whether he or she can do karma or not.
One can choose what karma to do but one can not choose not
to do karma. Arjun has the choice to fight or not to fight but by not fighting he can not
escape from karma.
As mentioned earlier one can choose what he wants to do.
However, that is not totally true.
Most of the vital work (karma) is not left to the
individual. The body breathes. This breathing is not left to the choice.
If it were left to human choice, the person might forget to
breathe, or may just not want to breathe. Several such vital actions are not optional.