When Sam went off to Stanford, the first thought that went through his mind when the bus pulled away from the curb was I'm free. Not just free of hunting, not just free of John's tyranny, but free of the ten-ton weight around his neck that was Dean's illness. Not that he was ever allowed to call it an illness to Dean's face. That was an easy way to get a tooth knocked out. No, it was Dean's thing. Or Dean's situation. The same situation that had turned Sam into a criminal at age 14. He still remembers the bitter taste in his mouth the first time he and John robbed a pharmacy to get Dean his meds. Sam had meticulously written down all the brand names (Depakote, Celexa, Seroquel, Wellbutrin) and then the drug names (divalprolex sodium, citalopram, quetiapine, bupropion hydrochloride), not knowing what they'd find on the bottles. They'd managed to steal about three months worth of each.
Looking back, that had been the easy part. It was much easier to disable an alarm system and pick a few locks than it was to get Dean to take the goddamn medicine. Dean had to be pretty damn low to consent to taking the pills, so they spent most of their time tricking him into taking them. Sam had spent the better part of 2 years waking up at the crack of dawn to crush up pills and open capsules to sneak medicine into Dean's oatmeal or orange juice or omelet. He's sure Dean must have figured it out, but food was food, and there was no denying that the medicine helped. Medicated, Dean was reasonable, logical, and patient. Unmedicated, Dean was a raging, miserable, destructive nightmare.
When they first put Dean in the hospital, when they first heard the word bipolar, Sam had gone online in the school library and done some research. He found out Dean's mood swings and rage were symptoms of a "mixed state" or "dysphoric mania", and for the first time he felt more sympathy than anger at his brother. Of course, he'd then gone home and had to help John fix the holes in the wall and the broken chair and he'd been pissed off all over again.
When Dean had come back from Hell, Sam assumed that Castiel had healed Dean's messed-up brain along with his body. Then he'd found Dean weeping in a hospital bed and realized he was wrong. Before they left the hospital, Sam tricked an intern into giving him a three-month supply of generic Wellbutrin and Celexa and samples of Risperdal, which his research has suggested might work better as a mood stabilizer than the anticonvulsants.
Once Dean gets out of the hospital, Sam shows him the bottles of pills and says, "Please take this stuff. You need it. It'll make you feel better." And Dean just holds out a hand and takes them without any argument. And up until Zachariah zaps them into Sandover Bridge and Iron, Dean takes the pills everyday without comment or complaint.
After the great Sandover experiment is over and they return to their normal lives, Dean shrugs off the pills. "Zachariah cured me," he says. "Told me to consider it his fee for the favor." And that was the end of that. After that, when Dean lashes out at angels and demons and everything in between, Sam doesn't have to worry about Dean coming home and destroying their motel room, then sitting in the middle of the wreckage and laughing his ass off about it. And then they go their separate ways, and Sam doesn't have to worry about anything Dean-related at all.
But this time, as the truck pulls away from the rest stop, he doesn't think I'm free. This time, he just thinks, I'm fucked.