Purity is discussed in this season by both Dean and Sam.
Sam talks about his deep need to become pure of his imperfections. The most important part of that is, perhaps, to be good enough to be approved of and seen as a person by Dean. Loved for his own self, rather than as someone to take care of. Seems that Sam believes Dean doesn't love him in that way, because Sam doesn't deserve it. As de_nugis
pointed out too, Dean doesn't give Sam a reason to think otherwise. Sam first confesses this at the church, to whatever deity. Later he confesses the same to Dean - who, for all he refused to a tool of heaven, sure developed clear moral ideas and expectations of Sam. And to Sam, growing up with this glowing, strong, self assured, beloved big brother, for all he doesn't know it - Dean is perhaps as close to a deity as he gets.
Dean talked about purity as well, mostly in the beginning of the season. To Dean, purgatory was pure. This can be read as merely an expression of things being black and white there, easy, with no moral dilemmas . If so, it should be asked how come, in this blessedly black and white setting, Dean decided to cross the line and become close to a vampire. Perhaps it somehow allowed it? Regardless, there's maybe another way of seeing this purity.
Purgatory is built almost exactly in accordance with glorified reports of WWII as a purely masculine space, in which men can closely bond in a way unmarred by sex and not disturbed by women*. In which men can revert to their true masculine form, discover what they're really made of, and kill plenty of monsters.
Perhaps to Dean, like Sam, purity is being worthy of love, to him by John? Perhaps being a soldier, like John was, killing monsters daily and without qualms, like perhaps John seemed to demnad, was a way of finally being what Dean imagines John wanted?
For all Dean is a baddass, he is not traditionally masculine in every way. He is vulnerable, at times soft. He is pretty. He is loving, sensitive, and often wears his heart on his sleeve. Even the way he allows himself to express delight is not manly. He takes care of people - which may have been appreciated by John when directed at Sam and himself, but perhaps less as a simple characteristic. At least, in the way Dean imagines what John would appreciate. In many ways, Dean is not a good example of traditional masculinity. He must know this. It's probably the reason for a lot of his overcompensation.
It makes sense that Dean makes such an effort to be "a real man", cause that's what he believes John wanted of him. What is was, perhaps, supposed to grow into by then, but hasn't because of some inner flaw . What he's supposed to be, to be worthy of being a person. Just like Sam towards Dean. Perhaps to Dean, becoming pure and worthy means scrubbing off the places in which he is not masculine enough.
*By the way, this resonates in a lot of SPN. For example - the tropes of women connected with this? The pure, incorporeal concept, often of a caretaker (like Mary, early Anna), the evil bitch (crossroads demon, Lilith and many others), the sweetheart at home who gets close to no characteristics and is not part of anyone's life (Lisa, perhaps Jess), and random one-night stands.