Because they are not grown up. It can take a long time for some people to accept full responsibility for their love lives.

Maybe because they don't want to take full responsability of their sentimental life... and their way to scape is blaming bad luck ... I'm just taking a guess here, each head is a whole different world and everyone has their own perfect 'good' reasons to do whatever they are doing ... ^^

They expect too much from one person. One person cannot fulfill every area of your life. Also women tend to give up their friends and hobbies just to concentrate to be with the man they love and when it ends they feel alone and sad.

because they do.