One possible reason could be Nero's over - eagerness or recklessness. Maybe Nero keeps getting into dangerous situations without proper planning, and Dante, being more experienced, gets pissed off at Nero's lack of caution.
In some cases, Dante could distance himself from Nero for a while. He might go off on his own to cool down and think things through. Meanwhile, Nero could be left feeling guilty or confused about why Dante got so angry.
The letter could potentially contain details about a Nero fanfiction writing competition. It could be inviting Dante to participate or perhaps it's just informing him that a story he might be interested in has won the competition. You know, it could also be a letter from an author who has written a Nero fanfiction and is seeking Dante's opinion on it, since Dante might be well - known in the fan community.
There's also a possibility that it's Trish. She could be calling to warn Dante about a new threat that she has discovered. Dante would trust her information and start to make preparations to face this new danger.
Well, in the fanfiction, perhaps it's a call from an old friend who needs his help. Dante might then gear up and set off on a new adventure to assist his friend.
Sasuke could be the cause. If Sasuke were to betray Naruto again in the fanfiction, it would surely make Naruto pissed off. Their relationship is complex and Sasuke's actions have a big impact on Naruto.
He might be excited. Dante is often associated with Nero in the same universe, so he'd probably be interested in what the fanfiction has to offer.
A new, powerful and arrogant villain introduced in the fanfic could be the cause. This villain could constantly mock Naruto and his ideals, and try to undermine his efforts to protect the village. Such blatant disrespect would make Naruto really pissed off as he is very protective of his home and the values he stands for.
Since 'lemon' fanfiction often involves inappropriate content, we should look at their relationship in a more general and appropriate sense. Dante and Trish have a complex relationship. Dante is initially drawn to Trish because she resembles his mother. They share a bond through their battles against demons. Trish also comes to respect Dante's strength and his sense of justice.
Another reason might be related to his family. In some fanfictions, his parents' story or his relationship with his family members could be distorted. Harry has a strong connection to his family, especially his parents, and any wrong portrayal of that could make him pissed off.
One possible plot could be that Naruto somehow gets transported to Dante's world. He has to learn to adapt to the new environment and use his ninja skills along with Dante - like powers he might acquire. They could team up to fight against a common evil that threatens both their worlds.