That's exactly what it is. Everything went into a box until Sara died and it stayed there and it let him care for her in all the hardest moments, even if he knew he couldn't carry the burden for her. Hopper knows he's not always the best man, wasn't the best husband, wasn't even the best father a lot of the time, but when she had been sick, she'd been his priority. The only thing he cared about.

Maybe that's why he'd lost Diane, too.

Nodding, Hopper smiles faintly, a twisted expression, and says, "And sometimes things stay in the box even after everything's done." He'd cried for Sara, held himself, curled into a ball and cried, but even after the grief had run dry, he had hurt. And so he had medicated himself with pills and with alcohol.