The movie is not about a love story, It's a tale that portrays different perspectives in relationships, the conflict of opinions that occur in any relationship. I would say unlike most of the happy ending gooey love stories, '500 days of summer' seemed very practical.
One person's idea of true love is not necessarily the others, so we need to respect our partner's decision, and if we don't want to go along with the other person's decision then it should be conveyed at that same moment.
In the initial days itself, Summer(Zooey) says Tom(Joseph) that she wants a casual relationship and asks whether he is cool with it, Tom agrees with Summer at that hour even he didn't want to, and thus he has to suffer once she goes. The movie also shows the pain one goes through after a breakup. and how to pull ourselves out of it. It teaches us that life goes on..and never to miss a moment, be a part of it what makes you happy!!!
And lastly, it conveys a message to the viewers don't go in darkness if your Summer is gone, because if Summer goes you will have Autumn.!!