yeah there are many reasons and also a thing called destiny sometimes plays it's role as well. Maybe some people are just not destine to be together
I don't think anyone can ever answer that question, until they are put in those shoes and experience the emotions, insecurities, pressure..etc...and even then, I believe that it's still one of those questions that has no concrete answer.
But I do believe that whatever happens, it happens for a reason. Yes, in that our hearts and everyone else who is involved has their heart played with