Why did him buying the lunch make any difference? If he didn't buy the lunch would he be less of a good guy?
But this is a researched social phenomenon known as the reciprocity effect. He's done something for you, so now you feel like you somehow owe him something in return. Often that's "paid back" by positive social feelings. I really hoped I could find a solid citation for you, but the basis of it is found in Robert Cialdini's excellent book "Influence: The Psychology of Persuasion".
[This is as close as I could get for sourcing, I apologize for the lack of quality on the subject: http://en.wikipedia.org/wiki/Norm_of_reciprocity]
If you want to make a friend, let someone do you a favor. -- Benjamin Franklin
But I need to point out there are huge cultural specificities here. In the US it is common for two unrelated (and even related!) people to split the bill. Hence, picking up the tab may have a positive social effect in the manner described because that runs contrary to expectations.
In other cultures it is traditional for one person to pick the bill. Either this designates social status (the most senior person pays) or a close-knit culture that ensures future reciprocity - eg. I'll buy you lunch today because I feel like it, and I know we'll have lunch many times and sometimes you will pick up the tab so it will all wash out in the end.
I meet and get to know a lot of people in my line of work. I've had the privilege to work with some tremendous folks.
Looking back, you try to spot patterns.
It's just that I've always thought fondly of the guy, more than all the other nice people I know, and never really thought about why. Reading that article made me think that he really knew how to enjoy himself. He knew how to spend money to make himself happy. And in return, he did this really cool thing for other people too.
It's not that buying lunch made him any nicer, I don't think. He's just a regular person -- not a salesperson or some charismatic person at all! He wasn't trying to game the system. He just learned that buying people lunch and stuff made him a happier person. And for the rest of us it had this incredible effect.
Offering to buy someone lunch can be a little awkward, especially in situations where it isn't usually done. My hypothesis is that they way someone pulls this off is the differentiator between whether it seems authentic or phony.
So it's not like he was going around asking to buy people lunch. We just had lunch and did stuff like anybody would.
You know, this is going to make me sound awful, but at the time I kind of thought of it as flaw -- it was even a bit insulting at times. He just wouldn't let me pay for anything. But he was that way with everybody.
It didn't stand out so much that I noticed it right away -- hell, it took me four years to figure it out. Just looking back, I don't think I ever paid for anything when we went out. And he did that to everybody. Four of us would go out for lunch, and there he was, insisting on picking up the check. At the time it was almost stressful, because I knew when the check come we would always argue over paying it. But it looks like long-term, he had a much more successful happiness strategy than my counting pennies!
The original post made it sound like one can be easily brought with a quick lunch. I personally am wary of having just any stranger do me favors because then I'd feel obligated when they ask me for something. This is how people end up buying entire rugs when they go to have 'just a drink of tea'. But yes, I understand this wasn't the situation you were talking about.