He's abandoned numerous promises to him, though. Without getting into the reasons why, some justifiable to my eyes and some not so much, he didn't make good his promise to look out for Dean like he said he would at the end of s2, he pretty much shat on Dean's dying wish of not using his powers by juicing the fuck up as soon as Dean was dead, I mean. I get where he's coming from, but Sam's word isn't worth all that much. In the end he's driven by what he wants, period, and always has been to varying degrees. It's just a lot more prominent now, b/c when he comes down from it, it isn't his brother he's going back to.