As much as it can be futile to try to find recruitment patterns in a manga that tends to embrace the random and weird, I'd still think that one major driving force of what leads Luffy to recruit some is how much he and the world-be recruit are emotionally connected. I think that very much shows during times like when Luffy uplifts their spirits during a point they're feeling very low or if Luffy deems someone as being crucial to his happiness, like Luffy being glad about Franky having helped rescue Robin, even though Luffy was mad at Franky for beating up Usopp.