What's happiness, love, or destiny?

Rating: 7/10 (9 August 2017)

I hadn't been able to understand this title, "500 Days of Summer", before watching this movie because I'd never thought "Summer" doesn't mean the season but a person's name This movie is about a man's growth through a woman whose name is Summer. I don't like her, though, because she looks like she trifles with his feelings.
I sometimes feel irritated and confused that the days skip rapidly in 500 days and their relationship changes significantly, but I also think that's a fresh and nice idea. It says it's not a love story, but it is. If we don't say it's a love story, what should be called a love story? It makes us think about what happiness is, what love is, or what destiny is.
Also, I'm really surprised at the ending. It got me! What's the next of Summer?