It's not just Pasoti though, it happens in life as a whole. Most people will complain from time to time about faulty products and poor service. I don't think many could honestly say that they go out of their way to praise good service as often as they complain. We tend to take the good for granted.
A lot of football fans are moaners - you hear it everywhere in the stands. It's a way of letting off steam and getting rid of life's frustrations. It may not be right, but it has always been the case.
There's also a much narrower potential for comment on a good win. You can say the team was great, X and Y were fantastic and the manager is a genius. Once 20 or 30 have said that, the thread becomes very repetitive. If you try to be constructively critical when the team has won, many object to that too. After a bad performance there is the potential to critically analyse, and many take that opportunity, often over zealously.
It'll always be the case that a poor performance draws more comment than a good one. Why worry about it?