To be honest, something I see a lot in fanfiction is the reasoning for Dream’s mask being about how Dream can’t control his facial expressions well and is too readable, making him vulnerable to people knowing exactly what he’s thinking and feels. Personally though, especially now that we can reasonably say that Dream is autistic, I think it’s actually the opposite.
At least in my experience, with myself and with other autistic people, often the trouble is making the appropriate facial expression with the proper enthusiasm. Like telling a joke and people not know if you are kidding or not because you say it with such a straight face. Something I really struggle with is emoting excitement and joy so I have to be extra conscious of making sure my face is portraying how I feel so people don’t get the wrong idea.
The other thing I have to be aware of facially is eye contact, making sure to do the proper amount and/or don’t look somewhere else that makes it seem like you aren’t paying attention. (This is something I’ve realized since diagnosis that I am good at but hate, which is why I plan socializing around activities so I can focus on that instead of looking at the person.)
These are just a couple of the things on my mind when having a conversation (along with body language, tone of voice, what I’m saying…etc), that can really change how people perceive you. When not executed well people can really get the wrong impression of you, even thinking that you’re super intense, serious, a bitch, cold because your facial expressions are so stern and neutral, when in reality you could be feeling a lot of things (many people in high school didn’t talk to me and when asked later they said they were intimidated by me, and I think this might be one of the reasons why).
So, because of this, personally, I think Dream wears a mask not because he’s too open, but because he doesn’t emote properly or just doesn’t want to bother to try and emote properly, and dislikes eye contact. To not have to focus on making proper eye contact and facial expressions takes away that factor of misunderstanding and just makes conversing a lot less work and more comfortable. With a mask on, he doesn’t have to work on that part of his social masking (;D lol), now he can look where he wants and just have a resting bitch face if he wants to without worrying about how it comes across. That’s what I think is more likely the reason Dream wears a mask because he is actually naturally less expressive.
Furthermore, perhaps if he lost his mask in prison then maybe that’s a reason people in the prison are generally more cold to him and see him as less human because his expressions are so blank, giving people the perception that he’s very cold. Therefore making it easier for Sam and Quackity and even Tommy to hurt and dehumanize Dream in prison because subconsciously they are put off by his lack of emoting, which only further adds to the idea that Dream is some embodiment of evil…