Sam can understand her feelings, even if he's died and been brought back more times than he cares to count. Though (so far) he's returned from death every time, that doesn't mean he doesn't feel left behind or even replaced.

He's quiet for a moment, reflecting on that to himself. When Dean was in purgatory and Sam did his best to move on, it had been hard, on a level that even throwing Lucifer back in the cage can't touch. Sam was alone, truly alone, and though that's all he wished for when he was younger, now it's the last thing he wants.

Does that make him co-dependent? A little. Hard not to be when his entire family has been taken apart by demons and angels alike.

"It is hard. It's not something you ever have to be okay with, though. I think you're entitled to feel that way, since it was just..." Here he trails off a bit, the memory of her and Ellen's death flashing through his mind. "It was too soon for you. For both of you. Seems to be a pattern when it comes to friends of ours."

In some ways, yes, Sam feels like it's his fault for letting them die. And on some level, he knows Dean feels the same. That ever present guilt, just for being Winchesters. The brothers handle it in different ways, but it's always there.