That's the thing about writing. the DS9 crew gets along just fine without a counselor for 6 years. Then, Ezri shows up and Sisko goes nuts. They invented "counselor stuff" for Ezri to do because her position couldn't be worked into the show without it.
They most certainly were not fine psychologically until Ezri showed up. They were probably the most psychologically screwed up crew of any Star Trek series, especially compared to TNG, where there were far fewer conflicts between the crew because they all got along so well most of the time. And I almost laughed at the line, "Ezri shows up and Sisko goes nuts". How many times did we see Sisko go crazy in the first six seasons? Too many times to count! That's why many of us (including myself) have criticized Brooks' performance for being too over-the-top sometimes - it's not just the actor's fault, because there were just so many scenes where he had to act like he was having a temper tantrum or a nervous breakdown and that's hard for any actor to pull off!