One possible reason could be his self - loathing. He has a complex past and has made many difficult choices. In fanfiction, the authors might explore his inner turmoil through self - harm as a way of expressing his pain. He may feel that he doesn't deserve any better because of his actions and mistakes.
Perhaps in the fanfiction world, the pressure from being a double agent during the wizarding war is too much for him. He has to constantly deceive both sides, and the stress and guilt associated with that could lead him to self - harm as a way to cope or as an outlet for his pent - up emotions.
One possible reason could be the extreme stress he endures. In many fanfictions, Snape has to constantly balance his double - agent role during the war. The pressure of keeping his true loyalties hidden from Voldemort and his Death Eaters while also trying to protect the students at Hogwarts can be overwhelming, leading to a collapse.
Isolation. Snape is a lonely character. He doesn't have many friends, and he is constantly in a difficult position between the Death Eaters and the Order of the Phoenix. His feeling of being alone and not being fully understood by anyone can lead to crying in fanfiction.
It could be the pressure of being 'The Boy Who Lived'. Harry has so much expected of him all the time, and it might just be too much to bear.
One reason could be that he realizes Harry has a lot of potential and wants to nurture it. He sees that Harry is in a difficult position, being the target of Voldemort, and decides to help him despite their past differences.
It could make some characters more sympathetic towards him. For example, if Harry Potter were to find out in the fanfiction, he might start to see Snape in a different light, realizing that Snape has his own deep - seated problems. This could potentially lead to a more complex and nuanced relationship between them.
One possible reason could be his overwhelming guilt. Snape had a complex past, being involved in the Death Eaters and then switching sides. He might have felt that he could never truly be redeemed, especially with all the lives lost during the wizarding war that he had some part in, whether directly or indirectly. This guilt could have driven him to such a desperate act.
Well, one possible reason could be a very extreme misinterpretation of Snape's hatred for Harry's father. If the fanfiction writer took Snape's grudge to an unrealistic level, they might write a scenario where Snape wants to destroy Harry as a form of revenge. But this is far from the nuanced relationship shown in the original books.
One possible reason could be extreme stress. Peter Parker has a lot on his plate, being Spider - Man and having a normal life. If in the fanfiction he faces a series of really tough situations like failing to save someone important or being hunted by a very powerful enemy, the stress might get to him and lead to self - harm as a way of coping. Another reason could be feelings of isolation. Even though he has friends, he might feel like no one truly understands the dual life he leads, so in the fanfiction this could drive him to such a desperate act.
One possible reason could be that fanfiction often takes creative liberties that may distort the complex character of Snape that J.K. Rowling created. Snape has a very intricate backstory and personality in the original series, and fanfiction might simplify or misinterpret it.
One reason could be to add a shock factor. It completely changes the expected family relations in the original story. Another reason might be to explore Snape's character more deeply. If Harry is his son, it gives a new perspective on Snape's actions and emotions towards Harry throughout the series.