The fragility of a friendship

We all fear that fragility in the friendships that we forge. The fear of having to invest in a friendship that was never meant to last. Is it fair to call it an act of hurt or is it just the integral way of nature that causes people to drift away from our lives. There are stages in our lives where we feel close to different people and when the idea of our novelty fades, so do they. Do we not have the right to expect a certain amount reciprocation of attention from a friend that we have invested quite a significant amount of time, resources and emotions into? Is that selfish to do so? Is it alright to feel hurt when someone whom you have considered as a confidant, someone whom you have poured out your deepest and darkest emotions to, someone who has seen your most vulnerable sides, someone whom you have shared your moments of happiness and sadness with, knowingly or unknowingly drifts away. Is it ever justifiable to ignore someone who decides to leave you when you need them most and decide to come back to you when they need you most? Is it ok to accept back a friend that had drift away and now expects to jump right back into the bandwagon when things aren't going well for them and they need that attention that they aren't getting from elsewhere anymore.
It is completely normal to have shifts in the hierarchial teirs of friendship. It is ok to accept drifted friendships again, but the place they hold in your heart might not ever be the same again.
No man is an island, no matter how strong someone seems to be managing their lives alone, it is never easy to completely be on your own and to not have someone to share your happiness with or share your misery with or to get advice on issues that aren't your area of expertise. That's where the dynamics of relationships one forges in their lives come into play. Not every person you meet is bound to remain in your life forever. They are meant to teach you a lesson that you are meant to learn. Humans are complicated beings on their own, when they are thrown into a mix of other beings and asked to interact, relate and bond, the complexity of those dynamics just explodes out of proportion. It is important to know oneself best and to know that the only one true being that one can always count on is oneself, and once we have come to terms with balancing our own complexities as a person, then only will we understand and accept the ever changing dynamics of the human relational experience and to be at peace with it.