David wasn't one to kiss ass, but with Roger, it was the complete opposite.
All the time, it was, "David, do this! David, do that!"
And the bad part, was that David would give in to his orders, just to try to get on Roger's good side. And it wouldn't really work; Roger would still treat him poorly.
David never understood why Roger treated him terribly. Roger would almost never treat the other members like that, so why David? David thought that maybe Roger treated him poorly, because he still couldn't get over the fact that he had replaced Syd. And David knew that Roger missed Syd immensely. The other boys knew that too. Roger thought that if he ordered the new guitarist around, he'd feel better, but at the end of the day, he was left thinking about the original guitarist, Syd.
Syd, Syd, Syd.
Syd was Roger's bestest friend. Since they were teens, it's been that way. It hurt Roger tremendously, to see how his best friend was turning out. He blamed it all on the drugs that Syd would take, even though he knew that there was actually something wrong in Syd's brain. He never wanted to admit that to himself.
So now Roger had to deal with Syd's replacement, David.
The boy was very talented at the guitar, and had a wonderful voice, but he wasn't Syd.
It took Roger a while to finally accept that.