When someone gives us something without asking for anything in return, the brain starts looking for a way to return the favor.
Remember your desk mate sharing his sandwich with you?
He didn't ask for anything.
But suddenly, you wanted to help him too, the next time he needed it.
Nothing changed.
Your client doesn't keep the conversation going just because you have a good service. He keeps it going more easily with someone who helped him before trying to sell him anything.
It doesn't matter how much you give away for free. What matters is whether what you give is valuable enough that people naturally want to come back to you.
Reciprocity