In my opinion, the good love stories are the least predictable ones.  They're the painful ones that have heartbreak and decisions and time people spend on their own trying to figure things out.  Some of the best love stories end happily, but some don't.  The best stories to me are the most complicated because they are the most realistic and raw.  Unfortunately, they're not realistic enough and still leave most of us confused whenever we find ourselves in similar real life situations and things don't go at all like we want them or expect them to.
The problem is that things aren't black and white.  They're much messier.  More people are involved.  More feelings are at risk.  In movies, people are oblivious to what they want, but in real life they want different things equally in different ways.  Decisions aren't just difficult but almost impossible.  In the real world, Allie can't just ditch Lon because her feelings for Noah are so strong because she loves Lon too, just like how Lon can't just let Allie go be with Noah with a smile because it's a love story and that's how its supposed to work out.
We are socialized to believe in the fairy tales and the bliss and the ignorance that everything we know about love tells us is true.  We are taught that when there's love and passion, there's enough to make things work.  But in the real world, it's not all about intangibles.  Simply loving someone is not enough to make a relationship work.  In a movie I recently watched, the girl made one insanely ignorant decision after another, then when her life was in shambles, her man took her back because he was just sitting there waiting on her because they were meant to be together.  Not only is that totally unrealistic but I believe that's undesirable as well.  No one wants someone who is that uninterested in building their own happiness, and no one wants to feel like someone else's last option.
There is a positive side to love not being what we thought it was cracked up to be.  We do get to find our own happiness.  We write our own stories and endings and can be our own Prince Charming.  We can have other relationships and life experiences and dreams and goals that we can accomplish and celebrate that don't have anything to do with love and relationships, and those things should be equally valued.  So even though everything is complicated and dark, there's still that bright side, and in my opinion, that's something great to celebrate.