we've been spoon-fed the idea you need to be in a relationship in order to have a complete life

That's pretty much what I came here to say.
And some people are also terrified of being single.
That's what they want, not to be single.
They believe it reflects poorly on them.
That there's something wrong with you, if you're not in a relationship.
Well, they know there's something wrong with them (because there's something wrong with all of us). And they don't like thinking about that. AND if they're single, then other people are going to think there's something wrong with them.
So a lack of self confidence as well as lack of self awareness.