Do not become attached to your bad feelings or to your good feelings for that matter. They are both equal in the fact they will both come to an end. Just experience things. The zen master was trying to tell the student that meditation isnt about good or bad. It's not about emotion.
Polarities seek their opposites.
A hot cup of coffee becomes cold if left to itself and a cold glass of water becomes warm. If you are currently in a rut, don't worry and be strong , it will pass', if your riding the pavement, however, beware it to will pass. Buddha-Sidharta is a river. The story is the true point of Buddhism. The only way to have a true understanding of the way we feel is to have an understanding that our emotional responses occur in a bigger picture.