According to Carl Jung, the famed Swiss psychoanalyst who did more investigation and study regarding the symbolism of dreams than anyone before or since, almost every aspect of our dreams represent a message for the conscious self.
Dreaming about old friends suggests that each of these individuals represent a specific aspect of your own psyche. The unconscious mind has fitted friends or foes into symbolic categories, which sometimes become enlivened when a particular problem is presented to us in our conscious lives.
As an introduction to the vast journals of Jung, start off by reading "Memories, Dreams and Reflections."
I have read probably read three-quarters of his works, and my mind has been expanded a hundred times. I can wake up now and know instantly what the "behind-the-scenes" story is trying to communicate ... at least most of the time.
What is interesting to me is that in my dreams I'm thrust into some kind of confrontation with an old girlfriend. Trying to figure out a meaning to our dreams is not always easy. I'm also particularly impressed when my unconscious mind creates a totally unknown female with whom I have a great attachment. (Read about Jung's analysis of the anima.)
I'd like for my dreams to always be about rainbows, flying with a great love via giant beautiful butterfly wings, but my dreams tend to be more problematic and filled with uncertainty. I have extremely few euphoric dreams compared to those that border on being nightmares.