I’m not the author here, but my understanding is not that you shouldn’t try to be a good, reliable friend. Instead it seems like he is suggesting we take a more mindful approach to relationships. Live in the present and don’t struggle against the flow, instead ride the current. Many of us tend to spend so much time trying to MAKE things work, we don’t realize that we would have so much more time for ourselves if we just let things flow naturally.
Example: I have someone who I care deeply for. I could think about how they could leave me at any moment, make myself constantly available to try and prevent that, pass up on other opportunities in case they need me, contact them throughout the day just to remind them I exist, etc. OR I could live my life, be the person I am, when they pop in my head, shoot over a message checking in, but then moving on to what I need to get done for the day. When I want to do something, I go do it, I can offer the person the chance to come, but the activity is for me. The more we grasp and try to hold on to things such as relationships, the more likely we are to smother it, or exhaust ourselves.
Let the gravity of your own existence bring people in and out. When someone leaves (and it’s clear that’s what’s occurring and that it’s not something you can work on), you could stress about it, wonder what you could have done differently, mourn for weeks; or you could look at the present moment and realize you are who you are, and you want to surround yourself with people that can handle that, not change yourself to something you think everyone else wants to keep people around.