The following is Ek Ong Kar Kaur Khalsa's deep break down on this teaching in the 29th paurhi of Jap Ji:
'sanjog vijog dui kaar chalaaveh lekhe aveh bhaag'
So as he does in so much of his poetry Guru Nanak is giving us the extreme polarities. There's 'sanjog' with the the total union, there's 'vijog' which is the absolute separation. Then he says 'dui kaar chalaaveh' or "both go along". In other words there isn't a stable point in life. We are always moving from union into separation, back into union. Because life is changing, we are growing, the universe is moving, we have moments of completion, moments of new beginnings, and moments of trajedy/challenge/separation, going into darkeness, coming out with new perspectives and going back into union. The Universe is going along between both of the forces of union and separation, and we are walking along with the universe.
If we want to understand out life, not to get in a picture in our mind of what is the safe, perfect life that is answered in the next words 'lekhe aveh bhaag' "our fortune and fate is already written".
So what Guru Nanak is teaching us is that our souls have incarnated for specific experiences. Those experiences are written for us before we even take birth.
So this is how we want to approach life: "Yes, I may have an idea of what I should or how it should be... but there is also a fortune written for me and I'm going to flow with what comes to be between these experiences of union and separation. In that process The Divine matures me, grows me. I am a Sikh, I am a learner, I expand and learn and grow through that process.
