As we've seen Dean is less of a thinker and more of an emotionally impulsive type so I don't think he was considering that these memories were just a slice of Sam's life. And these memories were pretty much the final straw--this has been building over many seasons--when it came to the tension over Sam leaving the family to go off to college. It really had less to do with Dean resenting Sam for wanting a normal life but more to do with Sam's normal life not including Dean. I think Dean felt that Sam saw a normal life as one not only being free from hunting and supernatural phenomena but one free from his dysfunctional family. And frankly, I think deep down in some ways that is true although Sam wouldn't necessarily acknowledge it.I just don't buy it, as it appears they are going through jumps from one memory to another, you would think Dean would be smart enough to judge that three memories don't make up a person.
Yeah but Dean usually (the exception of say Scarecrow after recently having him back in his life) represses all that, but its usually been threats that actually cause him to lash out. And if this had happened in episode 3, then I would absolutely buy it. As his abandonment issues had been really worked with Sam in the end of season 1 (start of season 2), with Sam being the one keeping them together more (certainly season three), but with both brothers pushing the other away during season 4, I can see them resurfacing. But Sam has been very open to Dean lately (for what based on episodes about 4 - 5 months), he's been the one waiting to be back with his brother (not Dean), Dean actually enjoying his time not having his brother with him.
Heck even faced with Sam's addiction to blood (and the bad memories that brings with it) Dean saw Sam being open and honest, and more even more so probably more drunk on power then ever before (certainly had more blood at one time then he has ever had), see him control his actions and work as a force of good.