In the very first episode of Season Four, after J/S get back (and ignore Daniel -- nice, guys, that's real sweet of you) and when J/S are in the debriefing -- they're talking and Daniel just has this... confused and almost hurt look. Like, "I'm right over here, what's up with you guys?"

Also, now that I'm thinking about it, there were some serious bitter vibes in his dream way forward in Absolute Power. I mean, he has Sam declared crazy and sent into a prison (and of course, I got the impression that he may have had Teal'c killed or something (because somehow Daniel got na... whatever in his system, right? Because he could use the whatsit), but he's probably bitter over the wife-killing thing with Teal'c). He's also very... cruel in a very nice way with Jack, giving him just enough rope to hang himself.