Arriving at each new city, the traveller finds again a past of his that he did not know he had: the foreignness of what you no longer are or no longer possess lies in wait for you in foreign, unpossessed places. Marco enters a city; he sees someone in a square living a life or an instant that could be his; he could now be in that man's place, if he had stopped in time, long ago; or if, long ago, at a crossroads, instead of taking one road he had taken the opposite one, and after wandering he had come to be in the place of that man in that square.
(Ítalo Calvino, Invisible Cities)
This is The Thing that gets to me whenever I look around (or back, or forward). Looking back means what if I hadn't...?, whereas looking forward means what happens if I...?. Looking around equals why am I not there, instead of here?, which road should I have taken to end up there?. And the infinite possibilities are mind-boggling.
If I believed in Destiny, I'd have the comfort in knowing that I'm here and not there because It Is Written. As I have no such belief, I simply wonder what past could've led me to different presents, what present will lead me to what futures.
This, mind you, is not a result of being dissatisfied with the present, but of mere overthinking allied to curiosity: did I sway when I should've swerved? Did I take the right when I should've taken the left? I don't long for different, but the Not Knowing what lay on the untaken roads behind me and what lies on all the roads I'll never take ahead of me... this is maddening. Although fascinating. (But that's madness for you.)
By now, from that real or hypothetical past of his, he is excluded; he cannot stop; he must go on to another city, where another of his pasts awaits him, or something perhaps that had been a possible future of his and is now someone else's present. Futures not achieved are only branches of the past: dead branches.
'Elsewhere is a negative mirror. The traveller recognizes the little that is his, discovering the much he has not had and will never have.'(ibidem)