In many ways, especially for someone like Luffy, fighting with a smile and a goofy arsenal of attacks is the ultimate expression of freedom. Plus, with the addition of Gear 5, it would be weird to ...
With Gear 5 as well, no matter how dire a situation gets, Luffy can’t help but laugh and have a good time. Of the many living things in the world, only a human being has the ability to smile!