I think relsqui got the difference. I will often not follow these guidelines when deciding when to offer help, or accept requests, just when to make them.

Just looking at the second paragraph, I don't think I agree with your definition of an unreasonable request. I think very few exchanges are really equal, but that doesn't make all interactions unreasonable.

In my mind, the stronger your relationship is, the more uneven the request could be. You could ask a stranger in the movies to perhaps remove their hat, but you can ask your best friend to watch your cats for 3 weeks.

I suspect that part of the reason it's okay to ask more of people with whom you have stronger relationships is that there is an expectation that they will at some point ask the same of you. The stranger in the movie you're never likely to see again. But your best friend will be around when YOU go out of town for a month. It's not really a "they'll be around to repay the favour" as much as "over the expected lifetime of the relationship there will be enough back-and-forth to come out even".