It's not so much that I would feel less self sufficient. I have been there, done that. I can be, and have been, self sufficient.
It's that I feel better knowing that there is someone to lean on if needed. And to enjoy things with, and has similar interests. To talk to when you need a sounding board. Even at 3 AM.
Someone not a blood relation, who doesn't have expectations of the way for you to behave. Who appreciates the things family takes for granted. You can be self sufficient, and still be in a relationship.
Everyone is different, some people prefer to be alone, and function well. Some function equally well, whether in a relationship or not. Some function best in a relationship, but are adequate if alone.
And some people have problems.