Earlier I took a look at the large and growing class of slang words that make it sound like you think everyone's going to call you a liar unless you explicitly tell them you're not. Honestly, literally, seriously, actually, I'm not gonna lie, and so on.
My interpretation was that these words have sprung up in response to the growing social and emotional distance among young people. If the listener doesn't believe the speaker to be trustworthy, then the speaker will have to make these elaborate displays of not being the boy who cried "wolf."
I still get that vibe when I hear young people talking like that. It's like "Hey, I know I don't normally interact with other people, and that you tend not to trust people who never open up. But I'm being honest here, and don't dismiss what I'm saying just because I rarely open up."
The more I hear these things, though, the more it sounds like there's something beyond an earnest appeal for the listener to give the speaker the benefit of the doubt. It's that, plus a demand to not judge or talk back. It's an attempt to shut down any possible argument before it gets started. It comes from the aggressiveness and insistent tone that people use.
"OK, honestly, nobody's going to want to marry me."
The girl who says this is also saying, "...and don't try to tell me otherwise." End of discussion.
"I am literally going to freak the eff out."
...so don't try to talk me into a more calm mindset. I will be freaking out, nothing can be done about it, so just stay out of my way.
Then why bother making it seem like you're opening up and initiating or furthering along a conversation, when you just want to shut it down after you've said your little piece? It's like their pseudo-slutty way of dressing and acting. "Omigosh, just because I'm wearing butt-sculpting tights and a cleavage-baring top doesn't mean I want to have to, ick, deal with boys."
They only seem to be interested in interacting with their peers. They just want to have their little message or signal recognized, and that's it. Just move on once their ego has been validated from getting a pellet of recognition.
Somebody here pointed out (in a comment that I cannot find) a similar thing about "So," especially as the first word after someone else has been talking. It cuts them off, ends their line of thought, and dismisses it all as trivial. It's like, "So -- now that you're done blabbing, back to me and my piece." If it's an argument, every micro-clause will begin with "so," as in "ergo," even if there's no logical progression. It's like, "Hey, I began with 'so,' hence you must accept it."
Or they put "again" at the beginning of every clause, as though they've already established or proven the clause as fact before. Or were you not paying attention? / did you not get it the first time? / is your memory so poor that you've already forgotten and need reminding?
"Honestly," "literally," etc. fall into this broader category of discussion-enders. They contrast the world of maybe / maybe-not with the world of established fact. Not hypothetically -- actually. Not figuratively -- literally. Not jokingly -- honestly. We're not in the world of make-believe, so you must accept everything I've said and can't argue back. It's not as though this were some kind of hypothetical debate, imaginative storytelling, or jesting and joshing.
Under this reading, the trend is part of the intensification of status contests. Every listener is a potential opponent and must be pre-emptively shut down. If they insist on flapping their gums, they must be immediately dismissed when you get the chance to butt in with "So." I don't think there was anything like this in the '80s, though. "For real" from the '90s is about as early as it goes.