Surprised no one's mentioned this but…
Physical activity releases Endorphins and in time of extreme stress, releases Adrenaline. Now, it's not to say Luffy is pure Adrenaline, but it can play a big part in someone's ability.
Like I've stated earlier, Luffy is a combat genius. A G-E-N-I-U-S. That's something of a rarity in the Shonen World. Most of the Shonen Heroes like Goku and Kenshin aren't very bright, but they train like mofos. Now Luffy is dumb EXCEPT for his combat ability.
Luffy's NOT the strongest person, but he can wear his opponents down, and that's what he does. Everything has a weakness. Enel was weak to Rubber because rubber is non-conductive. Crocodile was weak to liquids because they harden sand. Kuzan is probably weak to salt. And Ace? Well, water, obviously. The others Luffy has fought? Their weakness is their effort.
Zeff pointed it out when Luffy was fighting Krieg. If you go the distance without giving up, you will eventually win, unless you die. That said, Luffy will go the extra mile to find a weakness for his opponent.
Let me tell you right now, and feel free to rip this up: Strength has nothing to do with the ability to take a punch. Period.
Harry Houdini, considered the strongest man in the world of his time (Early 1900's), boasted he could take any punch. That's because he conditioned his body to withstand physical blows by posturing himself. That's a common technique. Well, one day, a man punched him unexpectedly. 3 days later, Houdini dies of a burst spleen. Tragic? Possibly. But my point is, being as strong as hell doesn't mean you can't go down to one punch. Krieg did, technically.
