Awkward reunions

Why is it that you can be good friends with someone – or at least have the ability to participate in light-hearted conversation – with someone, and then not see them for a while, and then, when you unexpectedly see each other again, it' is completely awkward and no one can think of anything to say? Am I the only one with that problem? Is that just me? Does my description of the scenario even make sense? Does this bother anyone other than me?