Over time, you may lose some relationships. Some will simply drift away with no real struggle while others come to a breaking point. In my life, I've had some lost relationships due to both situations. The relationships that drift away can be rekindled in some lighter ways. In these situations, time and events simply took us down different paths. With regard to relationships that hit a breaking point--those break-ups were more painful. In situations like that, the relationships simply took a negative turn that I could not understand and could not make better. While those relationships could be rekindled, the process would be a lot more difficult and likelihood for more pain is probable.
As much as possible, it is good to work for long lasting, positive relationships, but it's also okay to accept the fact that won't always happen. Onward.