Well, usually the fanfictions develop their relationship through a series of events. For example, they might be forced to work together on a difficult task at Hogwarts. During this process, they begin to respect each other's skills and knowledge. As time goes on, they start to have feelings that are more than just respect. Some fanfictions also explore how their past traumas can bring them closer. Snape's past with Lily and Harry's with his parents' death can be a starting point for them to connect on a deeper emotional level. And often, the relationship develops in a way that challenges the norms of the wizarding world.
Typically, 'Harry Potter and Snape love' fanfictions develop their relationship in a slow - burn way. At first, there's a lot of animosity or at least distance between them. But then, small things start to change. Harry might notice Snape's unwavering loyalty to the school or Snape might see Harry's courage in a new light. They might start having conversations that are more personal. As the story progresses, their feelings grow. Some fanfictions also add a magical element to their relationship, like a spell that binds them in some way or a magical object that helps them understand each other better. This gradual development makes the relationship more believable and engaging for the readers.