One reason could be a misunderstanding. Maybe some villain has tricked Hulk into believing Spiderman is a threat. For example, the villain might have faked some evidence to make it seem like Spiderman was planning something bad against Hulk or his friends.
Well, in some fanfictions, Hulk might physically stand in front of Spiderman to shield him from attacks. For example, if there are powerful villains shooting energy blasts at Spiderman, Hulk's huge body can act as a barrier.
Spiderman's agility and web - slinging abilities could be combined with She - Hulk's super strength. For example, Spiderman could use his webs to create a net and She - Hulk could throw it with her great strength at enemies. Or Spiderman could swing She - Hulk around to gain more momentum for her attacks.
He might also use his webs to his advantage. He could try to web up Hulk's hands or feet to slow him down. Or he could create a web shield to block some of Hulk's attacks. For example, if Hulk throws a big rock at him, Spiderman could use his webs to create a net to stop the rock.
A frequent theme is the idea of mentorship. Sometimes Spiderman, with his intelligence and quick thinking, can act as a mentor to Hulk in terms of strategy, while Hulk can teach Spiderman about raw power and courage. Also, the theme of protecting the city from common threats is quite common. They have to put aside their differences and work together to keep New York safe.
There could also be a plotline where they both enter a mysterious tournament. They have to fight other superheroes and villains in a series of battles. Along the way, they learn more about each other's capabilities and form a stronger bond.
One interesting 'Spiderman and She - Hulk' fanfiction could be one where they team up to fight a new supervillain in New York. In this story, Spiderman's agility and She - Hulk's strength complement each other, and they have to learn to work together despite their different fighting styles.
One common theme is the contrast between their powers. Hulk has raw strength and Spiderman has agility and webs. Fanfictions often play on how they can complement each other in battles.
One common plot device is a mutual enemy. There's a big bad villain that both Hulk and Spiderman have to face, and Hulk takes on the role of protecting Spiderman during the fight. For instance, a powerful alien invader.