Chapter 13 - 21

 For the spirit, abiding in material nature,    Experiences the Strands born of material nature;  Attachment to the Strands is the cause of his    Births in good and evil wombs.
The last two lines are most relevant here. Birth is not an accident due to intercourse between parents. It is an act of will by the soul which seeks sensory experiences in accord with its nature. We are having a life experience because we chose to, it is not due to accident or circumstance. The mere fact of having a desire, attachment implied that we had a will behind it. The life experience however good or bad is always the result of will, a desire to have a life experience.

Yogaredux: