That's truer in the old Lee/Kirby flashbacks, where Loki was constantly being played up as 'the Lord of Lies', and 'the root of all eeeeeeevil'. But there HAVE been a few flashback stories where we see Thor and Loki having a great time together. There was one - I think it was posted on here once, and if it wasn't it should have been - where a teenage T&L ambush a bunch of trolls lurking around the borders of Asgard. They're whooping and hollering and cracking jokes, and generally acting like two reckless teenagers having a good time together. (It's through Thor's perspective, and he actually says something like 'this was one of the best times I can remember.') Or, if you prefer straight mythology, there are LOTS of stories about Thor/Loki teamups - and not as kids, as adults! They work very well together when Loki isn't being played as the debbil in another form - Thor's the brawn, Loki's the brains. Oh, Loki's still untrustworthy, and he and Thor get into plenty of arguments, some of them very serious - like the time he chopped off Thor's wife's hair; that was an epic one - but by and large, they get along pretty well.