Summertime sadness for me sounds like short time relationship. It's like you went to a place for a holiday, met someone, gone on a date or what so ever and within a short period of time, you suddenly fell in love. After the holiday, you have to leave the place along with the memories that you have with that person and by the time that you have left the place, you'll never know if that person will have another lover of could have found a person that will actually stay for good. It's kinda sad if you're going to think about it deeply.