I'm feeling better now, LOL. I need to have a rewatch... I think Sam had to say this and Dean needed to hear it, but I'm also certain Dean's immediate reaction is that Sam is saying, I don't love you as much as you love me (or worse, I don't love you). On the back of certain scenes in Dean's history - like the YED inside his Dad saying "they don't need you, not like you need them", or Sam under the Siren's influence saying "you hold me back", and so on and so on...it's far easier for someone with Dean's self esteem issues to see this as a kick in the teeth from the person he loves and needs the most.

And we've already seen he's hitting the bottle again, even before this, just from Sam's we hunt but not as brothers (and the guilt over Kevin's death of course).