1. He Joined the Avengers Just to Assess Their Threat Level
For many a Black Panther fan the defining era of the character began in 1998. That's when Christopher Priest (once James Owsley) began his damn near mythical interpretation of the hero. One that even the Marvel movies are drawing from -- if just a little bit -- by introducing Bilbo Baggins as longstanding Panther sidekick Everett K. "They Took My Pants" Ross.
During Priest's tenure T'Challa was less a superhero, and more a super-politician (who just happened to kick the ass of people with names like Killmonger, and "The Devil" when necessary). Always seven steps ahead of his enemies, Black Panther was also nearly always just as far afield of his friends. Out of necessity, of course.
One such revelation early in the series' run put his relationship with the Avengers in a whole new light. Basically, Priest revealed (through the medium of Panther's political rivals) that T'Challa only joined the U.S.-centric big leagues to spy on them.
Faced with the existence of literal foreign superpowers his highness thought it was only "prudent" to investigate what kind of threat they might be to his kingdom. It was only over time that he came to accept Captain America and company as true friends and allies.
This revelation -- and the ensuing, amicable status quo -- was good enough for T'Challa. So good that he didn't feel the need to explain his actions further. Though it didn't necessarily sit well with the rest of the team. That mistrust was a driving factor in stories throughout Priest's run. A thread that culminated with the likes of Iron Man used as a pawn in one of his many plots.
The real kicker, though, is that T'Challa was right to be wary. As he almost always was during that era of Black Panther comics. Several international incidents (as well as misadventures involving time travel, mind controlling dragons, and cases of mistaken identity) later T'Challa came to blows with many a fellow superhero. Though he nearly always came out on top these victories didn't come without cost -- both physical and reputational.
2. T'Challa Turns Down the Illuminati, Then Starts it Up Again When He Feels Like It
If the introduction of The Illuminati isn't Marvel maven Brian Michael Bendis's greatest contribution to the comic book continuity it's damn close. The despicable council of Marvel's most arrogant, most patriarchal big shots set the stage for more than a decade of upheaval in the company's creative outlook.
And it wouldn't work nearly so well if it weren't for T'Challa -- a man who only appears in the first third of the team's one-shot debut issue, and then barely speaks. Though what he does say is pretty important.
"Walk away now," his advice concludes. At the time, of course, T'Challa was right. He saw the gathering of self-appointed white dudes (including Reed Richards, Charles Xavier, Tony Stark, and more) for what they were. Just as he saw his place in it as their token black participant. Rather than be a shade of justification in their little club, however, T'Challa did what he advised. The Black Panther walked away, and left the rest of the room to their bad choices.
Jump cut to that aforementioned decade later, and things have changed. Ten years after telling his supposed peers to go screw T'Challa became the driving force for The Illuminati's reformation.
Some might see this as an admission that the Black Panther was wrong, but really it only highlights one of his great strengths: patience. Bendis's iteration of The Illuminati only wanted to flex their culturally mandated control. Something they "accomplished" with world-class decision making. Such as blasting the Hulk off into space, and falling to infighting over the Superhuman Registration Act.
When T'Challa got the band back together during Jonathan Hickman's New Avengers, it was to prevent universal extinction. Rogue, parallel universes were bleeding through into our beloved Earth-616, threatening to crash, like, every party everywhere. T'Challa displayed one of his greatest characteristics -- absurd amounts of patience -- in bringing them together to combat an actual threat. Not to mention he got to do so on his terms, since it was he and his fellow Wakandans that discovered the threat in the first place.