Why have kids .. bring them up then constantly complain about them? Why?

Even when they are good to you and try their best .. yes everyone has their ups and downs but they aren't so bad to deserve this everyday criticism and complaining .. Even when they're doing something good .. their parents always seem to ask for more.. never satisfied.. Why are some parents like this ? I'm asking to be honest not complaining about them.. I'm just a bit baffled.