Well, change is a big common element. People change over time, and in first love, sometimes one person changes faster or in a different direction than the other. And then there's the lack of communication. When couples in first love are afraid to talk about their true feelings, problems can build up. These two things often lead to the sad endings we see in first love stories.
In sad first love stories, a common factor is external pressure. For example, family disapproval can put a lot of strain on the relationship. If parents don't like the partner, it can create a rift. Also, differences in future plans play a big role. When one person wants to travel the world and the other wants to settle down immediately, it becomes difficult to continue the relationship. All these elements can make first love end in sadness.