Nathan Drake is ex-military.Nathan is unreasonably good with guns, hand-to-hand combat and keeping his cool in a gunfight for someone who is supposedly 'just' a fortune hunter. Considering his dedication to the treasure hunting trade both for the money and the adventure, it's not unreasonable to assume he enlisted, probably under the Army or Marines, as a grunt, to get physical conditioning for work in the field, build up capital to finance his operations, and to learn how to handle himself in combat, knowing that rival fortune hunters might very well shoot at him before anything else. Notice how he doesn't seem to owe anyone money like Sully, nor does he tire quickly or complain about the lack of amenities while on the job. It would certainly explain his complete, hard-line refusal to leave a man behind in Among Thieves.
Uncharted 2: Among Thieves is a prequel.In all of the promotional material that's been released so far, Nathan is not wearing Sir Francis Drake's ring as a necklace. Since the necklace was returned to him at the end of the first game, and since Sir Francis Drake was revealed to be a hero for dying to protect the secret of the island from getting out, you would think that Nathan would continue wearing the necklace as symbol of pride. If, however, Among Thieves is a prequel, it is very possible that Nathan isn't wearing the ring because he hasn't found it yet, and at some point during the game he will acquire it, realize what the numbers written on the ring mean, and decide to check it out, setting into motion the events of the first game. In this case, it would go without saying that the expedition of Among Thieves is going to be a financial bust, because this would explain why Nathan is bothering to chase after treasures "after" the expedition to the island, why he and Sully were hoping that the treasure of the first game was finally the real deal, and why Sully and Elena are not in Among Thieves (not that we know for sure that they aren't, but Naughty Dog hasn't confirmed that they are, and there is a new female love interest, so at this point I'm assuming they're not in).
Nathan Drake hooked up with Eddy Raja's sister.