“Any fool could count the seeds in one apple, but only Hashem, the highest power could count the apples in one seed. ” ~ Facing Adversity with Faith by M.L. Cramer as quoted by Rabbi Paysach Krohn who explains that when you do something for someone, you plant a seed of friendship and you have no idea how those seeds could blossom.

Here is a link to the shiur if you are interested: http://www.torahanytime.com/audio/giving-and-getting-a-two-way-street/