No need to have any reason. For remembrance if you want to give a reason. Easier if you go to a place where the pp dont speak English e.g. in places like Bali or France, all I did was smile and point to my camera to indicate that I wanted to take a picture - use gestures. Sometimes it works sometimes not.
Its easier to take pics of pp I meet in the service line - like the gnarly captain of a ferry or the chef of a pastry shop, those would be easy as you are technically a customer. When I mentioned some pp use gifts, those would be useful if you wanted to take pics of street pp or performers - u literally donate money and then ask for a pic.