Well, if you're paying attention, you may have noticed that something isn't quite right with Will Byers. Apparently all that time in the Upside Down really screwed with him, and now he's having frequent flashbacks that put him back into the parallel dimension, where the Demagorgon and all kinds of monstrous, vine-like shadow tentacles live. Following the bad advice of his mom's new boyfriend, Bob (Rudy!), Will turns and faces the giant, ominous creature that's been haunting him. That plan backfires, and he gets shot full of Upside Down tentacles.
Starting to sound familiar?
Will begins behaving strangely -- the warm bath Joyce draws for him is too hot, he says, but then cryptically says it likes it cold. He eventually begins to lose his memory; in effect, he is becoming "no longer Will." And that's in many ways more terrifying than losing a loved one in an accident. Had Phineas Gage died, for example, his family likely would have been grief-stricken, but the pain of seeing a loved one return as a different person altogether can inflict a longer, deeper kind of trauma on those who knew the person before.
In this way, Will's initial diagnosis of post-traumatic stress disorder, and the repeated references to it, draws a similar parallel that makes Stranger Things more contemporary than it may appear at first glance. We know now that soldiers returning from combat, for example, may have escaped without any physical wounds, yet have unseen mental trauma that can cause long-term damage and even change their personalities permanently.