analysis of the saying,"if there's any good thing i can do let me do it now for i shall never pass this way."

Answer 1

There's a word missing.

"If there's any good thing I can do let me do it now, for I shall never pass this way AGAIN."

It is a prayer, or something a person says to himself, and means that the opportunity to do some good is often transient...like helping a hurt person by the road...if you pass by intending to come back, the person may die or (hopefully) be helped by someone else who will get the psychological or spiritual benefit for helping someone.

Answer 2

First off, that isn't the correct quote. The correct quotation is:
"I shall pass this way but once,any good,therefore,that i can do or any kindness that I can show to any human being, let me do it now. Let me not defer nor neglect it, for I shall not pass this way again." -Dale Carnegie

Second, it means -- if you can help some one, or do something good, do it. You won't be in this exact place again.