Paul D isn't an easy man to get to know. (Although if you want to try, take a look at our "Character Analysis" section.) How do we know that? Because he keeps going on and on about his tobacco tin box self, which—in male-speak—basically means he's a hard, self-contained, cold, detached, and unable to commit to a woman. Long enough list for you?
The tin box signals privacy and secrecy. But not in a safe-feeling way. Boxes, in general, aren't a happy place for Paul D if you consider that he was locked up in a box when he was on the chain gang. Just like the coffin-box Paul D experienced in Alfred, Georgia, the tin box isn't even a thing Paul D can open by himself: to get at his real self, he needs the women around him—both Beloved and Sethe—to show him who he can be.
It's easy to credit Beloved for opening Paul D up (yep, through sex), but we've got to give some props to Sethe, who starts the whole process:
By the time he got to Ohio, then then to Cincinnati, then to Halle Suggs' mother's house, he thought he had seen and felt it all. Even now as he put back the window frame he had smashed, he could not account for the pleasure in his surprise at seeing Halle's wife alive, barefoot with uncovered hair—walking around the corner of the house with her shoes and stockings in her hands. The closed portion of his head opened like a greased lock. (3.119)
Sure, he doesn't actually mention the tin box. But Morrison probably just thinks you're smart enough to figure out that the "greased lock" is an indirect reference to our symbol.