I can't argue further about Napoli and Insigne so I'll just leave it at that.

Your other point is just meh. Football in Spain is not 'enjoying' the game. Football in Spain is passion and emotion. This means that people get angry, happy, emotional, devastated, hateful and everything in between with their players and other clubs. It's part of the experience and nobody 'gives a bad image of their club' by whistling lol. Yes, perhaps here for people like you on Reddit but nobody in Spain thinks less of Valencia.

Besides, there is a lot of politics involved between Spanish fans and clubs which therefore adds more oil to the fire. It's generally just not accepted to join Barcelona if you are a Valencia kid, almost similar to how it's not accepted to join Real Madrid if you are a Barca kid. Perhaps you are new to Spanish football and think Valencia is a midtable club, but they are not. Valencia are historically a Spanish giant and will always be considered that way.