Everyone understands sadness, it happens to be a fact of life that everyone must experience it at one point or another in their life. Love is a different story altogether. Yes, everyone must also learn this via self exploration, but it doesn't mean we understand it. Love is a foreign thing and there happens to be no definition of it. So, what happens when we put the two together? A heartrending love story: a compelling twist of emotions to create just the right combination of events to make the reader experience it through their eyes, to feel the emotions themselves. Now, the question on your mind is: how do I create a sad love story?
Since love is so very unknown to the human race, it leaves a lot to be discovered and therefore, it's hard to write about. However, if you have experienced this feeling for yourself... Absolute adoration of another, the undeniable inability to go on without them, the want to spend the rest of your life with this person... It becomes easier to capture in sad love books.
Sadness is common. It is a fact that life has turned into a rule. When writing your story, you will want to remember this: without sadness, then there would be no happiness... There can be no highs without first getting low.
Bring your main character down to the brink of defeat, and at the last moment, create the climax of the story by bringing together the last thing that would actually happen and build him/her to the brink of ecstasy.
Romeo and Juliet: Shakespeare's story of star-crossed lovers doomed to be apart for life. Without the other, both are lost and their only goal to be always together. When messages go astray, things become unclear and conclusions are jumped to.
This inevitably results in the lovers deaths and a heartbreaking love story. Romeo saw the sleeping Juliet he surmised dead, and took his own life. When she woke up, her Romeo had died and the poison was her only salvation, for a life without him is void and impossible.
Each sad love story must have an equal amount of sadness, happiness, and tragedy. It just so happens that humans find the unlikely to happen, appealing. To put it very simply: use your imagination in a way that would involve making a tirade of emotions swirl in the readers mind.