Like any group of friends, the Scoobies bonded because of a certain set of circumstances, in their case the double bond of high school and patrolling the hellmouth. Once that particular set of circumstances stopped, the individuals began to form their own separate lives and to drift apart. We see this begin in Season 4, when part of the bond is severed for Xander. He goes out into the "real world" and joins the work force, while Buffy and Willow remain students. The group still has the hellmouth, but, beyond that, their lives begin to diverge. By Season 6, Xander is a full time construction worker, Willow has entered into the equivalent of marriage with Tara, Giles has gone back home to England, and Buffy is dead. Buffy's resurrection brings a temporary re-bonding of the group, but it is only short lived, lasting through Season 6 and partway into Season 7, but "Empty Places" was the death knoll for the original Scooby gang, and we will almost certainly never see the group that close together again. Certainly, they may come together in a time of crisis, Twilight, for instance, but once the battle is over, they all once more go their separate ways.
That's not so much being "bad" friends, or even losing the feeling of kinship that they have enjoyed for all the years. It's just growing up, growing apart, and each making his or her own life. The closest thing to the "gang" still in existence is Spike and Buffy, and even their bond has evolved into something different than it was in Sunnydale.
