Why would a stranger pay for my drive-thru meal and just drive off?

Why do people do that? I pulled up and the drive through lady was like your meal was paid for. I was like huh? The girl in front of you paid for your meal. Oh... thanks. you would think I would do that one day, but im too selfish I guess lol I get the whole giving thing, but I mean they just drive off before you even can say thank you. Last time this happened I was digging through my change to get rid of it and I pull up and they tell me the person in front paid for your order. I guess they think I'm broke lol

Why would a stranger pay for my drive-thru meal and just drive off?
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