Its a combination of things but the vast majority of us do it. I really couldn’t see many fans singing Klopps praises were he a side we have disliked for years’ manager e.g. Man U. Add to that if he was bringing them success, would any of us really be happy and not showing signs of jealousy or downplaying achievements, he’s amazing but he is OUR manager. Is it really suprising he gets a lot of criticism while managing us? It happens to everyone to be honest and most football fans dont give a shit about some of the more important things, there’s no superiority, it seems to be the same across the sport. It’s like the top players, if the top players leave your competitors you enjoy it, the focus isn’t what they bring to the league, you like seeing the chances of your sides success increase and that of your rivals decrease. Peoples arguments for measuring success never make sense either 
the moving of the goalposts is always going to be the thing
Best thing to do is just ignore the nonsense and focus on enjoying the fact WE have Klopp as our manager, it’s pointless to expect some adoration from the other sides because he’s a direct rival and we would to be honest be the same
Football support is still stuck in the dark ages a lot of the time and the arguments are very childish, to expect more is to set yourself up for disappointment. They can say what they like, we know our level and we’re doing things many sides haven’t ever experienced, that brings hatred, jealousy, downplaying and whatever else you want to call it
I can only speak for myself here, but I like to give credit when it's due.
I find quality, authentic human beings to be thin on the ground, sadly. But Klopp is one of them. I'd recognise that even if he managed a football rival like the Mancs or a local rival like Everton. I'd still loathe United if they had him, but I couldn't loathe the man or what he achieved there.
I was never jealous of the Mancs when they had Ferguson. I acknowledged what they achieved and I'll always say he was an immense manager. I've never liked the man, though. I respect his achievements, but the man not so much. Same with Clough. A great manager with many amazing achievements in the game, but an absolute twat of a human being.
I was never remotely jealous of Everton in that brief window in the 80s when they were magnificent. Kendall did a great job for a time. Credit where it's due. I never knew enough about him to like or dislike him.
Of course, we all downplay to some extent. It's easy to rile a Manc by saying they were battered in Barcelona and only played for two minutes when they won their treble. It's also easy to say they won leagues with piss-poor points totals and there was no real opposition back then. All that can be a laugh when it's not really being serious. The thing I find with us though, is so many actually believe their downplaying mental gymnastics. They really are serious, and they simply refuse to give any due credit. Even worse, it's not just fans either, it's often media professionals.
