If we can give Pay It Forward props for anything, it's that the title clearly illustrates what you're getting yourself into: A saccharine-soaked storyline unapologetically laced with a morality lesson.
We all know we could be kinder, more selfless human beings, and thanks to Trevor (Haley Joel Osmont) we know how. We only need to do something nice for three people, and those three people will then do three nice things for other people and trigger a widespread disease known as altruism. We'd elaborate, but we can't because we just vomited in our mouths a little bit.
It's not that we're cynical (we are) or that we know this simplistic plan would never work in real life (it wouldn't), we just resent that after we're force-fed this optimistic philosophy (Osmont literally draws the whole plan for us on the blackboard) and forced to wade through wholesome sentiment before we're accosted by a surprisingly horrific conclusion that shows the idea's collapse. Which goes to prove our point, that blind kindness is dumb, and should be reserved for people who deserve it, not indiscriminately parceled out to strangers.