When you’re long in the tooth enough, you should know what it feels like to pick up on old friendships and associations after being apart for a long time. In my experience, there are no guarantees that the relationship dynamics will be the same as they were back in the day, so it makes sense to be cautious about relying too much on anyone having your back, based on what happened in the past.
One issue is the fact that the time spent apart has taken each person involved in such a reunion on a personal journey and there is no way that one person can fully understand the state of mind that the other one has reached, neither can he or she work out exactly how that point of view was reached.
This is not to suggest that reunions should be avoided. Sometimes, the relationships can resume and pick up a new form of impetus, especially if there is good chemistry between the people involved.
Maybe it is important to enjoy friendships and other relationships as much as possible in the moments that they occur, seeing as one can never tell the way things are likely to turn out at a later stage.
It seems to me that folks from some cultures are brought up with the idea of sustaining friendships much more than they are in others. I learnt this particular lesson the hard way.