It'll make your head explode to find out that giving someone a B+ in that situation is more likely to lead to an A the next.. positive encouragement leads to positive feeling about the task. It's bullshit to think some kid is gonna think "oh I did bad I'm gonna do more of this to get better.". it's really easy to see how "oh I got this, I'm going to do more of it"

(This works for subjective learning only. Things that require mastery like mathematics needs a different system)