No short answer for this one...and I think somewhere deep down, you know that.
Each person probably needs to change in some unique way - might be that someone believes they need NOBODY and they secretly fear they are really TOO needy so they detach from everyone to never have to deal with that. OR someone might always be terribly NICE to everyone and never feel anger, maybe because they're afraid that deep down they have murderous rage, so they avoid the entire anger issue by just submitting to everyone around them and becoming a victim.
Each person has gotten STUCK in some self-lie (or likely, many of them). We go into therapy and want the shrink to fix our symptoms, but dont' want to touch the "way we are" - we say "no, that's not a problem for me, I just want to change THIS"
Can't have that.
Symptoms are systemic, they're related to the entire STRUCTURE of the self we've created over the years. We can't just tune up PART of self without really being willing to rehaul the entire engine. And we're petrified of that, so we stay stuck.