@josie, your friends and family know that you already had the virus and presumably won’t catch the virus again. People you pass in a store don’t know you. They don’t know why you aren’t wearing a mask, and there are vocal people in this country who have decided that they will simply pout and whine about the whole thing and refuse to wear one because they can’t be bothered to go to minimal effort to protect themselves and their community.
When you walk around without a mask, you look like you’re one of those people—others you pass in the store don’t see someone who’s already had the virus and who believes they cannot pass the virus on; they see someone who doesn’t give a f*** about passing the virus along. Wearing a mask is just basic courtesy right now. It’s communicating to others the thing that is already your position—that you take seriously everyone’s role in not passing the virus along.
And by contrast, by wearing a mask (even if you believe you can’t spread the virus anymore), you are actively encouraging others to wear a mask, too. There are surely people in your area who are potential carriers/transmitters of the virus and who aren’t wearing a mask (since it doesn’t seem like it’s required when entering stores where you are.) Be part of the effort to make wearing masks normal, so that others wear them as well, because you know that if others wear masks it will make a difference, and you also know that people don’t want to stick out (one way or the other). If you’re a badass, be a mask-wearing trailblazer.
But I’m just repeating in different words what others have already said. If you really won’t wear one for basic courtesy, then will you wear one for your own self interest and the interest of possibly at-risk family members? Although it sounds like a reasonable assumption that having antibodies protects you from the virus, it’s not a sure fact yet:
“The World Health Organization has also said that ‘there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection.’ Until more is known with certainty, it is thus best for recovered patients to continue avoiding elderly people or members of other susceptible groups.” (source for that quote, though this statement from the WHO can be found from many sources.)
(And as @ARE_you_kidding_me has mentioned, a well fitting mask does offer some level of protection to the wearer—more or less depending on materials, fit, etc. And when everyone is wearing a mask, it provides much better protection for everyone.)