I think Thanos manipulates Loki remotely with the Mind Stone, that explains why Loki looks so pale at the beginning of the movie. It's just a theory.

Loki wanted earth not the scepter, and Thanos is known to be ruthless. Even the ravengers were scared of him.

It isn’t a good idea to piss him off. Loki just does it to Thor because he knows Thor won’t hurt him.

I honestly think it was a deal where thanos sent loki to earth with the mind stone to aid loki to get the space stone. Then loki gives both the mind and space stone back to thanos and in return, loki becomes the king of midgard (earth)

Thanos entrusted the Mind Stone to Loki. However, he later learned better to not trust people like that.

To help Loki succeed. High risk, high reward. Thanos wants six stones, he has one. He lends Loki the Mind Stone so that Loki can use its power to help acquire the Space Stone; after Loki conquers Earth, Loki would have to give both stones to Thanos.

Unfortunately, it being a high risk strategy, Thanos not only failed to acquire the Space Stone, he lost the Mind Stone in the process. Of course, that's the in-universe idea; the real world explanation being that the MCU hadn't thought of "Mind Stone being inside the Scepter" while making The Avengers.

Guys, I see all of you making comments about "he had a deal to give him the 2 stones afterwards" but seriously, Doesn't Thanos know Loki is the god of Mishief and always betrays people???

@SwiftBlade10 Loki was told in Avengers 1 that if he didn't return with the Tesseract: "There will be no barren moon, no crevice, where he cannot find you." It took Thanos six years, but he did eventually find Loki.

Thanos did give him a warning that he will find him and he did and killed him in avengers infinity war

@SwiftBlade10 Thanos didn't watched the Thor movie