It really surprised me that we didn't hear Dean say he couldn't stay in the Djinn's wish!verse because he couldn't leave Sam (the real Sam, out there in the real world looking for him) alone - but then I had some time to think and it actually fits.
(Not that I don't believe that was part of the reason he came back, he must have been feeling it a little, he just didn't say it out loud)
We all know Dean has MASSIVE self-esteem issues and I don't think it's until later series that Dean has started to realise/accept that Sam needs him just as badly as he needs Sam. So back in the second season, even with the whole; can't-let-anything-bad-happen-to-Sammy-don't-let-him-go-darkside thing Dean didn't even consider that it might be his death that would be the thing to push Sam over the edge.
He genuinely believes that Sam would be fine without him, that he could carry on and go and live a normal happy life.