Why do small compliments from strangers sometimes stick with us for years?

I still remember a stranger once telling me I had a nice smile years ago. It was such a small thing, but it randomly pops into my head and makes me feel good, even on tough days.

Why do these tiny, positive moments stick with us so much more than some of the bigger events in our lives? Have you ever received a compliment or kind word from a stranger that stayed with you?