I remember when I first got into Self-Determination Theory (SDT), it was the two factors "competence" and "autonomy" that made the most sense to me and that seemed to play into each other in a logical way. Firstly, SDT is a psychological model about three factors (or "needs") that create motivation in an organism, but to me, it's a much broader model about what makes a healthy organism. The three factors are "competence", "autonomy" and "belonging" ("relatedness").

"Competence" is the organism's need to exercise their innate capacities (and fulfill their evolutionary needs, either directly or by proxy): for example, a cheetah running to catch their prey, a bear catching salmon in a river, an amoeba following a trail of food, an Olympic spear thrower throwing their spear, or a musician playing their instrument. Expressing your competence feels inherently pleasurable, meaningful and valuable.

"Autonomy" is the psychological side of that. It's the organism's need to subjectively experience that they're able to be who they want to be, not what somebody else or something else wants them to be. This naturally leads to acting in a way that is consistent with their innate capacities, because your innate capacities is in a real sense who you truly are, thus expressing who you are (autonomy) will naturally involve expressing your innate capacities (competence). In other words, you naturally want to do what you're good at (or what you're "made to do").

When something interferes with the expression of your innate capacities (through "extrinsic motivation", e.g. being forced to play a music instrument as a kid), you tend to dislike it and experience less motivation, because innate/intrinsic motivation is the strongest kind of motivation there is, and again, to deny somebody's intrinsic motivation is to deny who they intrinsically are.

So those two factors seem to fit together quite nicely and in a logical way. Now, I've always struggled with how to make "belonging" belong to those other two. Here is a definition from Wikipedia: "Will to interact with, be connected to, and experience caring for others". It seems a bit odd and forced to be put in there, and it seems to only apply to indeed social animals. "Well, we need a social aspect in there as well, so why the hell not?". That's what I thought when I first learned about it. But of course, at least for social animals, it makes sense if you look at the fact that being social seems to be an important aspect of health and evolutionary fitness. But it doesn't seem to have the same level of logical connection to the other two factors, at least on the surface.

But just today when watching this video of John Vervaeke (requires membership to iai.tv), the pieces suddenly fell into place. He talked about belonging in this way:

We usually think of evolution as organisms being shaped by their environment to fit that environment (which again creates their innate capacities). But in reality, there is a co-creation going on: organisms also shape their environment ("niche construction"). In humans, this becomes very clear when looking at society and culture. It's essentially niche construction gone wild. It's when social animals come together and develop language, technology and other new ways of shaping their environment. The last part is the most important: we are each other's environment, and we're shaping each other. That is what being social means.

So what is belonging then? Belonging is when the innate capacities of the organism fit with the environment. There! It actually fits perfectly with the other two factors, and now it explains belonging both in a human social sense and a more general non-social sense: Why do you gain health, strength and love from being with your family (most likely)? Because you're most likely very similar. You share the same innate capacities, and you're therefore each other's well-fitted environment. You belong to each other. Why does sharing similar interests with your friends feel so fun and meaningful? Because you share similar capacities and act as each other's well-fitted environment, thus you belong to each other. Why does being deeply open and honest with your partner feel good? Because you act as each other's well-fitted environment and thus belong to each other. Why are you here, together with people who share the same interests, same life goals, same worldviews, same outlook on life? Because you belong here.
  And of course, even the amoeba can be said to belong to their environment if their environment best facilitates their innate capacities (e.g. just the right pH level, salinity, food availability). Again, it doesn't have to be just the social environment, but it's just that the social environment makes it really clear. And again, I really do appreciate how this definition ties all the way down to the base biological level, even unicellular organisms, just like the other two factors do (if we assume amoebas have a subjective experience, which you can argue they have). It shows the scientific and metaphysical depth of the theory (which you could say resonates with my innate capacities, or at least interests ).   What this has made clear for me (but which I've also intuited for a long time) is that I need to find environments where I belong, particularly occupationally and interpersonally, but also more generally. Finding your strengths, your passions, your dreams is one thing, but finding the environments that resonate with that is another. That requires effort and vision, unless you're lucky to already be in that environment. Maybe you have to create some of that environment yourself (like we've always done as organisms ), or maybe it's waiting for you somewhere. Whatever is the case for you, it's something to aim for.

It has also emphasized for me nuances like why there could be cases where it's reasonable to say leave your family, even though that would seems like a big loss on a social level, because you might just not belong there (e.g. it's a toxic environment for some reason). That said, I've never thought that leaving your family was an absolute no-no, but this new framing makes it even more clear why it's indeed not an absolute no-no. But of course, that shouldn't really be on most people's minds (I hope). It was just a thought related to some earlier discussion.

That's it