I also think the chemistry between ZYL and SW is one of the reasons why Guardian works so well. There's a lot of things unsaid between them that the audience can see in all the looks they exchange. There's a lot of focus on the characters' reactions to each other and it's done without altering the pace of the story. Even if it could be argued that SW is also some kind of cold character, as is LWJ, we can see from the very beginning that he is holding back, that there's a lot of shit going on under his distant façade. With LWJ, we have to guess (through his brother's subtle teasing or whatever), he makes it a point to be inexpressive. It's more difficult to establish a visible bond that way, I think.