Well, Rob and Scott were friends earlier on. Dean entered the picture later, for whatever that's worth.
With that said, and correct me if I'm wrong, Dean had his own drug issues. I never heard that about Robert. Perhaps Dean was Scott's "buddy" in that regard, so that's where the resentment came from - as in "hey, who are you to rip me on my using when you do it too".
Again, I don't have any inside information, but that's how I've always figured it to be.
And yes, according to reports, they almost came to blows at the end of the SLDD tour in 2002.