Tony Stark/Iron Man has been the face of the entire MCU. Time and again, he has proved that he is in a constant battle with himself, to better himself and protect the Earth. Since the first Avengers, when Earth faced a galactic threat for the first time, Tony Stark has been the leader of the superheroes.
Sure, he made some mistakes (Ultron), but he did it in order to prevent the worst that he dreamt of. At that time, he didn't know the name - but in Infinity War he found it.
Talking about Thanos and Tony Stark on the commentary track of the Infinity War digital download, the writers and directors revealed the connection between these two characters. In this way, both of them try to prevent a terrible future from happening.
However, their paths cannot be more different.
According to writer Christopher Markus, both Thanos and Stark are futurists. But, if Stark doesn't know the threat, the Mad Titan knows who the leader of his Earth-based enemies is. And he has known this since the failed invasion of New York.
“You realize this interesting parallel between Thanos and Tony. They’re both aware of something from an early point and constantly having to deal with being smarter. Thanos is a futurist as much as Tony." (via Comic Book)
It was Tony Stark who delivered the finishing blow to his army. And that is why he has respect for Iron Man.
This is also the reason why the Mad Titan wants to kill him. After killing Loki, the death of Tony Stark is the only death he is interested in personally. And that is why the moment when Stark is stabbed is even more shocking.
Tony Stark is at his best in Infinity War, having developed his nanotech suit. But still, this just keeps him in the fight against a villain who has shed his armour. So this is another interesting parallel between the two characters.
If Stark continues to develop a costume to protect himself and Earth, Thanos has a different kind of quest. According to Joe Russo, the Mad Titan is a holy warrior and only Stark can threaten his agenda:
"It’s interesting because, as Thanos moves forward from this moment, once he disposes of his armour, he almost becomes a holy warrior where he doesn’t spend a lot of his energy intentionally trying to murder people unless they are, in some way, a threat to his agenda. Almost no one ends up being a threat to the agenda except for Tony who I think he feels has a sort of existential connection to Thanos.”
