It could be that Draco has done something to harm Harry's friends. Harry is very loyal to his friends, and if Draco does something to endanger or hurt them, like getting Hermione in trouble at Hogwarts, Harry would be deeply affected and might break down in tears.
Maybe Draco says some really mean and hurtful words to Harry. For example, he might make fun of Harry's family situation, like his parents' death or his relationship with the Dursleys. Words like that can be really cutting and could easily make Harry cry.
A possible reason could be that Harry is feeling the weight of his responsibilities. He has to constantly fight against Voldemort and protect the wizarding world. When Draco sees Harry cry because of this, it shows the toll it's taking on Harry. Another reason might be a personal loss. If Harry loses a friend or a mentor, Draco could witness his grief.
In many fanfictions, when Draco sees Harry cry, it can be a really powerful moment. It might be a turning point in their relationship. Maybe Draco realizes that Harry is not just the 'Chosen One' but also a vulnerable person. This could lead to a new understanding between them, and Draco might start to see Harry in a different light, perhaps even developing feelings of empathy or something more complex.
At first, it would become more strained. Harry would likely be angry and hurt by Draco's actions that made him cry, so he might avoid Draco even more than before. He might also be more on guard around Draco, not trusting him at all.
One possible reason could be his Slytherin pride and the long - standing rivalry between Gryffindor and Slytherin. He might have said something mean to Hermione in the heat of that rivalry. Another reason could be jealousy. Maybe Hermione is achieving more academically or getting more recognition, and Draco, being a proud pure - blood, can't stand it.
One possible plot could be that Lucius is disappointed in Draco for not being as cruel or cold - hearted as he wants him to be. Lucius might try to toughen Draco up by putting him in difficult situations, like making him face a powerful dark creature alone. Draco, being scared and not ready for such a trial, starts to cry out of fear and a sense of being let down by his father.
Perhaps in the fanfiction, Harry accidentally reveals a secret of Hermione's that she didn't want anyone to know. This could be something deeply personal or embarrassing, and the shock and hurt of it being out in the open could make her cry.
It could be that Sakura betrays Naruto's trust. For example, if Naruto entrusts Sakura with a very important secret, like something related to his family or his new power he's developing, and Sakura reveals it to their enemies or uses it against Naruto in some way, that would be a huge blow to Naruto and could lead to him crying.
One common theme is rivalry. Since Harry and Draco are often at odds in the original story, in these fanfics, the rivalry can be pushed to the point where Draco is so frustrated or defeated by Harry that he cries. Another theme is pride. Draco has a lot of pride, and when Harry shatters it in some way, it can lead to tears. For example, if Harry is better at something Draco values, like magic or popularity.
In the fanfiction 'The Lost Friendship', Harry makes Draco cry when he chooses his other friends over Draco in a crucial moment. Draco had always secretly hoped for a real friendship with Harry, and this rejection is too much for him. Also, 'The Harsh Truth' is a fic where Harry, in an attempt to make Draco see the error of his ways, tells him some brutally honest things that make Draco realize his own insecurities and he ends up crying.
Maybe Naruto discovers some dark secret about Tsunade's past that she has been trying to hide. For example, something related to her lost loved ones. Naruto, in his usual straightforward way, confronts Tsunade about it without realizing how much it would hurt her, and thus makes her cry.