I wonder about the exact relationship between Thor and Loki throughout Thor: Ragnarok, they're like fire and ice. What makes Thor keep trusting Loki despite all the times Loki has cheated his trust? To think of that, in Avengers Loki was the super bad villain, and in one way or the other Loki has always tried to overthrow Thor for the throne. But almost every single time Thor keeps acting like Loki is not the God of mischief that he is. Any thoughts?
In the Marvel comics the first incarnation of Loki was pure evil. Modern comics brought him as a much more complex character, one who wanted nothing but to be loved by his father.
His people have always treated him as an outcast. Marvel Loki is pretty much like the Loki in mythology: he's bound to be a villain. The difference is that he does not enjoy it. He hates it but he can’t help it. He tried to change (Kid Loki) but the fate (and the company as well) does not allow him to change. He's bound to be a bad guy.
But there are signs of sibling love when Thor and Loki have fun doing the 'help' gag in the elevator and furthermore Loki shows signs of goodness when he had the choice to rule the planet of Sakaar but instead he chose to fight with his brother and save the people of Asgard by not only bringing the ship round with warriors from the Contest of Champions, he also trusted Thor by bringing back Surtur when Thor stated
it's not about stopping Ragnarok, its about starting it
Other accounts Loki changing and trusting Thor's judgement is on the spaceship back to Earth. Loki questions Thor asking him whether it's a good idea to go back to Earth but ultimately trusts Thor.
Thor even gives a speech (probably trying to persuade Loki to join him) stating that he "always dreamed of fighting alongside" Loki but Thor simply puts it
you are you and I am me
In conclusion, Thor as you say 'keeps acting like Loki is not the God of mischief' because instead of treating his brother like an outcast he envisions him and his brother fighting together conquering all their enemies (he clearly states this in Thor: Ragnarok in the elevator scene); similar to Odin and Hela when they conquered the 9 realms.
Thor did not trust Loki, and knew he was the God of Mischief. He told Loki in the elevator scene he'd be better off staying there, because no matter what Thor wants he knows they are what they are. He also predicted Loki turning on them by putting the restraining device on him.
But at the end of the movie he has more willingness to trust Loki, because Loki could have done anything he wanted but chose to take the fighters to Asgard to help.
But in a more general sense, Thor wants them to truly be as brothers and fight together. He holds out hope that Loki can overcome his ways and be an ally and friend. After all, they grew up as brothers (probably over hundreds or thousands of years too).