Perhaps he leaves for personal growth. America has become a place full of painful memories for him, like the loss of friends during battles that happened on American soil. So, he decides to start anew in another place.
As he leaves, he could face challenges on his journey. Maybe some villains try to take advantage of his vulnerable state during the move. He has to use his wits and some basic tech he has with him to fend them off.
One reason could be that he's frustrated with the leadership or the direction the Avengers are taking in the fanfiction. He might feel his ideas are not being heard.
Maybe he doesn't fully understand his powers yet. Tony Stark is a scientific genius, but new powers might be beyond his current understanding. He could be worried that if he reveals them too soon, he won't be able to control them properly and might cause harm. So, he chooses to hide them while he studies and figures them out.
One reason could be the loss of a close friend like Steve Rogers. Their complex relationship and the times they fought side by side, if Steve were to sacrifice himself, it would be a huge blow to Tony. He might cry out of grief and also regret for the times they had disagreements.
Tony Stark may dislike Avengers fanfiction because it can mess with the emotional core of the team. The real relationships between the Avengers are deep and complex. Fanfiction might simplify or distort these relationships. For instance, his friendship with Steve Rogers has many layers, and fanfiction could make it into something that doesn't do justice to their real bond. Also, he's a man who values his privacy to an extent. Fanfiction might delve into his personal life in ways that he doesn't appreciate, making him feel exposed and misrepresented.
One possible reason could be a major disagreement. Maybe the Avengers made a decision that Tony strongly opposed, like a dangerous mission plan that he thought was too risky. For example, if they decided to team up with a morally ambiguous character without proper precautions. Another reason might be personal issues. Tony could be going through something in his personal life that makes him want to distance himself from the group, like dealing with the stress of his identity as Iron Man being public and feeling the Avengers draw too much unwanted attention.
One possible reason could be that Shield has been corrupted from within. Maybe Tony discovers some dark secrets or unethical practices within Shield that make him believe it's a threat that needs to be eliminated. For example, if Shield was involved in illegal human experimentation or was secretly working with enemies of the state.
One possible reason could be that Tony Stark is being overly critical of Peter's superhero work. Maybe Peter made a big mistake during a mission, and Tony's harsh words in his attempt to correct Peter made him cry. For example, if Peter accidentally put civilians in danger and Tony scolded him severely.
Guilt. For example, if he fails to save someone he cares about. He'll feel responsible and that could lead to him crying.
In fanfiction, emotional distress can also be a factor. If something really bad happens, like the loss of a loved one or the destruction of his work that he has put so much effort into, the shock and grief could be so overwhelming that he faints. Additionally, he could be suffering from some hidden illness that flares up suddenly and causes him to faint.