We do know from Oden’s flashback that Blackbeard never sleeps, a quick rundown on clues given before.
Luffy mentions that people in snow countries can’t sleep because they’ll die if they do so, and in the same arc we hear that the Blackbeard pirates attacked Drum Island.
Later we see a picture of a young Blackbeard crying at night, and finally we see the confirmation in the flashback (note here that this was right after the Whitebeard vs Roger fight ended).
So far the speculations were about how his strange body made him unable to sleep and how that would kill him, but this post isn’t about his special body, instead: Blackbeard literally cannot allow himself to sleep in fear of being killed if he does so.
One key word that really pops up a lot when Blackbeard is the focus in a chapter is “Survival”, here right after he exchanges some blows with Ace he tells him to join his crew since it’d be “a shame to waste his power” which the latter refuses by saying “I won’t live with regrets”, Blackbeard then mentions how survival is all that matters.
Look at his reaction to facing possible death, very much unlike the D’s we’ve seen as he clearly fears it possibly because he wants to survive.
The way he chose his crew members was by taking the survivors of the all out brawl on Impel Down, note that in a way, the same thing could apply to Shiryu as he survives imprisonment and the guy says something about waiting for the kind of man Blackbeard is (same ideology of survival for the fittest probably).
He also tries to get powerful people to join his crew (Ace, Moria) and seems to always gauge other people’s strength, he first does so with Luffy on Jaya with the Haki/bounty hunting thing, tells Bonney how she’s too weak to be a potential crewmember and makes a simple comment about how Shanks and Buggy don’t look too tough in the flashback.
He wants the most powerful people he can get and memorizes all Devil Fruit shapes from the Encyclopedia just so he could get the Yami Yami no Mi which he calls the most dangerous Devil Fruit in history, and the Gura Gura no Mi which was dubbed one of the most powerful Paramecia (SBS vol.58) all so he can raise his chances to survive.
Even his first ship was a Raft, the classic need-to-survive boat.
Now with that in mind, here’s the theory part:
Blackbeard grew in a country where he knew nothing but constant war and where, as said before, even one moment of carelessness like sleep means death.
What he says when he gets Whitebeard’s fruit, he mentions “boring peace-loving citizens” first among all.
There was some speculation about Drum Island reminding Blackbeard of home but what exactly does?
Here’s Drum Island with its Drum Rockies and after the Sakura “trees” are planted.
This is the biggest clue as to why Drum reminds Blackbeard of home, remember when Franky proposes to build the Sunny? He starts off with the talk about an island with a never-ending war that has the HUGE Adam tree which never fell.
Now there are speculation about Adam trees being on Elbaf and the fact that no snow is shown on the tree above, but Franky also says that there are a few trees like that in the world which implies more than one island have said trees.
Blackbeard’s home island is the one Franky mentions, it has one of those Adam Trees and is the reason Blackbeard attacks Drum Island because the Drum Rockies + the snowy country reminded him of his homeland.
Theory by Nanto_Suichoken (https://www.reddit.com/user/Nanto_Suichoken)
