I think it's saying that it was summer, and they both met eachother, and didn't mean to but they feel in love. They were so focused on eachother they didn't realize that they had to leave eventually, and when the final days came, they tried to have it be normal, but they couldn't. And when they left, it was over, all because summer just had to end.
I had this happen, and I sent this song to him, I cried for like an hour.