One possible element is the setting. A gloomy or restrictive setting can enhance the feeling of sadness in the love story. For instance, if the story is set in a war - torn area or a strict, conservative society.
The use of symbols is also likely. A withering flower could symbolize the fading love. Or a broken object might represent the shattered relationship. Symbols help to convey the deeper meaning of the super sad true love story more subtly.
The characters' flaws can be another element. If the lovers have personal flaws that contribute to the downfall of their relationship, it adds to the tragedy. For example, one might be too jealous or the other too indecisive.