I do think Sam has suffered the more personal losses than Dean has recently. Eileen was, arguably, closer to Sam. Cas "died", but came back, so he's not a loss per se. Dean accepted Jack, Sam really cared for him. Mary is important to both boys. Over the years, Sam's emotions have been buried in part because Dean goes into hyperdrive about how bad he is feeling, so Sam just internalizes, I think this has been one blow too many. I think Sam is feeling guilty because he's more vulnerable emotionally AND because he couldn't express that vulnerability when Dean was falling apart.

Plus Sam has been the one blamed for things for most of the series. Sam's remote from John and Dean? He's the one who left, not they're the ones who never tried to contact them. Lucifer is released, everyone remembers who broke the last seal, but no one really brings up breaking the first seal OR the 64 other seals, Does Sam even know the angels were letting that happen, because Zachariah only told Dean, IIR. Sam comes back without his soul, and he gets to face all the awful things he did without it. It looks like Dean has died, when he only went to Purgatory, when he comes back everyone jumps on Sam for not looking for a dead man. The Darkness gets released, it's Sam's fault for trying to remove the MOC. The pattern is to let Sam know when he has failed. By now, I think he sees every failure as his responsibility.