You know if someone you don't remember from an interaction ages ago emailed you to say "Hey, you probably don't remember this, but you once told me $thing, so I took $action, and now I've achieved $outcome. Thanks you for that, it really helped my career!", it'd make you feel great and would probably be the highlight of your day, right?
I remembered the person, but not anything about what I'd said that day. I assume it was just what I normally say. Having that last realization made me feel even better.
The easiest, most rewarding thing you can do daily in your life is appreciate the people around you. We all need that feedback.
I'd like to second that. Do it!
A few years ago, through some bizarre corporate scheduling (and turnover) accident I ended up interviewing almost all prospective interns and university hires at $work for a couple of years. I must have talked to a hundred second, third and fourth year students every spring, and at least twenty or thirty other people throughout the year. I definitely don't remember all of them, although I sure wish I did.
Since these were literally people who were still in school, I always gave detailed and constructive ideas for improvement. I always suggested things to read or what projects to look into, even to people who did very well on the interview and ended up being my colleagues. It wasn't some grand gesture -- these were young people who were still in school, so all I had to give were entry-level suggestions (read this chapter in this famous book, try this program, work on your understanding of these concepts) that any experienced professional in my field should be able to give with zero effort.
Lots of these young folks wrote me back after some time (mostly via Linkedin but one of them specifically came at the office to see me when they got hired by a company which had offices in the same building). Every time I see one of these messages I get the happies, even if I have no memory of ever talking to that person.
(Edit: FWIW, I often don't, but I understand the asymmetry here. Lots of us remember the first few interviews of our careers.)
The software industry has fewer and fewer islands of sanity and human decency. If you ended up on one, you should make friends with everyone you can, and if you appreciate something they did for you, you should let them know. The professional climate of our days rewards narcissism, arrogance and pettiness to such a degree that lots of things that would otherwise look tacky are viewed in a very different light. As the great philosopher and musician Sting once put it (maybe he stole it from somewhere though?), at night, a candle's brighter than the sun :-).
This also made my day because I saw the real-world impact of something on HackerNews. And perhaps it will make your day to know your comment has influenced more people than the poster you were replying to.
If he didn't remember you before, he'll remember you after. I've given thousands of semi strangers advice in various forms. I remember the two or three who followed up with me and said, "thank you for your advice, I followed it and have since reached my goal," and have kept in touch with them. It makes you feel really good to hear that.