So this is my theory (which is most likely a joke) that Jango is the reason for Luffys "Immortality and Strength".
In the battle against Captain Kuro's men, Jango hypnotizes his crewmates to get stronger, have their wounds heal, and get stronger still. During this he also unwittingly hypnotizes Luffy. Jango never undoes this hypnosis, and therefore Luffy has had this boost the entire time.. haha
In conclusion, Jango is the secret behind Luffy's "immortality" as he always heals from everything. And behind his amazing ability to always get stronger.
In the battle against Captain Kuro's men, Jango hypnotizes his crewmates to get stronger, have their wounds heal, and get stronger still. During this he also unwittingly hypnotizes Luffy. Jango never undoes this hypnosis, and therefore Luffy has had this boost the entire time.. haha
In conclusion, Jango is the secret behind Luffy's "immortality" as he always heals from everything. And behind his amazing ability to always get stronger.