Warning: contains spoilers for Avengers #50!
Marvel's Thor is unique among the Avengers in that while the other members of the team are enhanced humans, the God of Thunder is indeed a God, and hails from a realm full of Gods. Asgard is often depicted in comics (though frequently the visual look of the real is highly dependent on the artist), but only recently has Marvel confirmed what the realm actually smells like. In Avengers #50, readers get their answer - and why Iron Man will never willingly visit Thor's ancestral home.
In Avengers #50, She-Hulk is back to her old self (and her old body) after shaking off her Russian reprograming. The so-called World War She-Hulk having come to a close, the Avengers take stock back at their headquarters. Captain America, Captain Marvel, Robbie Reyes (Ghost Rider), and Iron Man all wonder exactly what is currently going on with Thor, who is absent from the meeting.
In past issues, Thor fought a battle against the Russian gods, Perun and Chernobog, who are currently recovering at a hospital in a Wakandan prison. The team wonders if Thor's "hammer problems" have anything to do with his current mood (Mjolnir has been quite fickle as of late, with Thor being unable to lift it but people like Loki can scoop it up without a second thought). "Worse. Family problems," says Iron Man, who plans to talk to his teammate. "Man, I hate going to Asgard," he ads quietly. "Even the air tastes like beer."
Thus, Asgard smells like beer - which Iron Man despises. As a recovering alcoholic, Tony Stark has learned to handle his addiction by staying far away from alcohol of any kind. Traveling to Asgard, then, would be the worst possible decision for Tony to make. He does put his friend's well-being above his own, however - but the fact that the realm of the Gods reeks of beer means that most of, if not all the Asgardians have a terrible habit that Tony has spent most of his life trying to break. As drinking is culturally different on Asgard than on Earth, Tony might not even be looked upon as a friend if he refuses one.
Asgard smelling of beer does make a sort of sense when one knows the Norse Gods' legend of Valhalla. In the afterlife of a warrior, all of Odin's finest soldiers would battle every day and feast every night. With that in mind, Asgard would be a paradise for Thor, but a nightmare for Iron Man.