Reddit reviews On the Self-Regulation of Behavior

Carver and Scheier for life, baby!

I also consider myself blessed for having been exposed to Bill McGuire's perspectivist approach to theory. "The opposite of a great truth is also true."

Great question, OP!

This answer is a bit more of a "how" than a "why", but maybe it'll spark some interest anyway...

This book describes us as behaving in ways that imply that we have a hierarchy of behaviors structured under goals and ideals. Any behavior that doesn't contribute to some higher-order goal or ideal will tend to feel less rewarding, so you'll do it less... and to not have goals or ideals at all or the ability to set them and hold to them (or to organize your behavior as if you did, since sometimes they're subconscious) can be dysregulating. Purpose and meaning are in some sense even bigger meta-goals and meta-ideals, ways we can organize our lives that are bigger than an individual person, sometimes spanning longer than any one person's lifetime. In that sense, we seek purpose and meaning because it sucks not to have it. It leaves you blowing in the wind, like everyone in this thread is saying.

To make this more concrete, here's a clinical example of something that could be related: people with borderline personality disorder tend to have many short, intense relationships that flame out, coming in part from experiencing a strong conflict between wanting to feel close to people and being overwhelmed by fears that they'll be hurt or abandoned. They also tend to experience an unstable sense of self. The unstable sense of self might contribute to overly intense, conflictual, unstable relationships by robbing them of a stable backdrop against which to ask themselves "who am I in this relationship and what do I actually want it to be for myself?" In other words, I think those behavioral phenomena might be a particularly vivid example of what happens when you don't have access to some organizing principle to hold sway over your fleeting whims, habits, less helpful or savory subconscious motivations, etc.

If it's purpose or meaning that you feel conflict about or a lack of rather than a sense of self, that's not going to produce the same surface manifestation of unstable relationships... but it might well produce a life course that has the same overall structure of instability, maybe manifested instead in ways like not being able to pick a career or a college major, demotivation leading to cycles of procrastination and then hyperproductivity once the pressure is past a certain threshold, making major changes like moving to a different state often without having a clear picture of what it is you're looking for, that kind of thing.