Too Much Investment in Social Capital?

I am baking a cake and have run out of sugar, but I can go to my neighbor next door to get some. It was worth being nice to that neighbor even though I did not particularly fancy her. Do I have to return the sugar? Maybe she can borrow my lawn mower the next time she needs to mow her lawn and that will count as a return of the favor. Maybe the value of the sugar is trivial enough not to require repayment in kind. Now I need a recipe for a new cake. My neighbor has just the right recipe for me. But there is really nothing to return except for good will because in giving me a copy of the recipe my neighbor still has it. A guy three houses down the street whom I do not know has heard from my neighbor that I know something about computers and asks me to help him. I am busy but feel obligated to at least try to help because we all live in the same neighborhood. Someday I will have to ask a neighbor, maybe even a different one, for help in fixing that darn lawnmower. What goes around comes around. In fact, I am really in a bad mood because the cake was not really any good, the new recipe was of no great help, and besides, my lawnmower is busted.