Because when Neymar does flair-moves, it's offensive and he must apparently accept the risk of getting attacked for it, but when the ever smiling and humble Ronaldinho does it, it's art.
The level of double standards...
You need a full context.
It is a double standard, but Neymar "earned" his status/hate with a lots of different episodes over the last few Seasons.
Look at it this way:
1. there is one guy who punches you and provokes you every day, for 300 days in a row
-- then you lose your nerves, and you say to yourself: the next time when he'll even look at me, I'll punch him in the face. I have had enough of his crap
2. and you have another guy, who is always nice and friendly with you and never caused you anything bad
Then you meet those 2 guys.
1. guy 1 pushes you in the chest again, when you meet him
-- of course that you will punch him, for "accumulated" sins and for today's sin
2. guy 2 pushes you, and you won't punch him
-- you won't do anything or you will ask yourself: wtf, what's wrong with him today?
So, the full "context" is important.
The guy 1 in this story is Neymar.
The guy 2 in this story is Ronaldinho or Messi.
For example, no one ever wanted to fight with Ronnie.
Or Messi, he can dribble past 8 defenders, and no one will say anything bad about him. And not to mention that someone will want to punch him.
And then, there are some guys whom more or less everyone wants to punch, guys like Muller, Robben, Neymar etc.
There is a reason and a long history of accumulated sins, as posted already:
He is our player, but I would punch him so hard after this crap
We can love our team and our players, but let's face it, they can be a real idiots and extreme duchebags from time to time.
And pretending how we don't understand why everyone jumps on Neymar (and not on Xavi or Rakitic, lol) all the time is just very biased...
And NO, the answer is NOT: Everyone hates him because he is such a good player
(How come that no one ever was punching Puyol?
And how come that Puyol was never blowing kisses towards opponents..?)