It is "social struggles", its not just one specific "thing".
We are very direct thinkers and so we might lack subtlety. Some of us don't process what we hear or see in person to person to person interactions as fast or maybe as well as everybody else in the room.
We might not recognize when people are offended, when they are angry with what we are saying, or that they are trying to end our sometimes (many times?) single minded conversations about our favorite topic. (these are examples of how our sensory processing can miss social cues).
Because we have experienced a lot of rejection (many of us) we might be extra sensitive and be hurt or quick to anger, or both when no offense is intended.
We might not have anything in common with many others so it can be difficult to find common ground to build a relationship on.
It is not just one thing, but a very wide range of different "things". For many of us it can be a lot of things!