I think Dean agreed to let Sam take on the Mark because he wanted to respect Sam’s decision. Having said that, I 100% guarantee you that Dean assumed he could somehow save Sam before the Mark seriously affected him. Because “we can fix this” is an essential part of the Winchester DNA. 

I was searching for something yesterday and this is what came up. It’s like some kind of hilarious quiz where you have to find the answer that’s wrong.

Supernatural monster problems: You know, Sam and Dean would be so much easier to deal with if they weren’t joined at the damn hip 24/7.

(Jump the Shark 4×19)