My good friend Kate has, as always, cut straight through my flailing and over-analysis and provided the obvious answer.

It’s a matter of TIMING.

A portal out of Purgatory - unlikely to be something you can repeat, yes? Dean and Benny opened the portal, they and Cas were all set to pass through. Out of the blue (or perhaps orchestrated by Benny?) Cas gets jumped and pulled back. Dean tries to grab him, but time is running out, the portal’s only open for a limited time, if he doesn’t go through then he’ll never escape (or so Dean thinks).

He’s not going to be able to free Cas in time.

The choice is either let Cas go and escape. Or run back and save him, leaving them both trapped in Purgatory forever.

And there’s a LOGIC to Dean’s decision, isn’t there? At least this way one of them gets out, at least one of them has a chance at a real life again. And if Dean runs back, ending his chance at escape forever, there’s not even a guarantee Cas will be saved. Whatever has him might be something Dean can’t defeat, and then he’ll be trapped forever. So logically, LOGICALLY, using a soldier mentality, putting emotion side - leaving Cas is the best option.

Yes.

Cas is the comrade who fell during battle.

You either leave him for dead or you die together.

Dean chose life. He chose, in fact, the VERY SAME THING Sam chose when he left Dean for dead. Oh Dean, you and your hypocrisy. He saw enough - enough to make the call. Cas was done for. No point risking himself and his future for a lost cause. But Sam saw enough to make the same call. And yet what they both saw wasn’t the truth. Their perception was wrong. But does that mean their decisions were? Rather depends on YOUR perception of the situation, doesn’t it?

Yes. Yes. THIS is how it went down and how it’s going to play out. I’m calling it.