You can see everywhere that haters or trolls roam free on the Internet. There aren't really that many rules on the Internet. It is just restricted by the website rules, but you can get around them quite easily also. Everything in life has at least the "haters" club.
What are haters? They are basically those that hate that particular thing strongly or they are just trying to mess around with you... which is a troll.
Haters in general hate because of these probable reasons.
1. They really dislike the show and they just want to express opinions (which is fine)
2. They want to make other people angry.
3. They are ignorant.
Let's take example Nintendo to Microsoft and Sony.
The Wii U first is announced by Nintendo during E3 2011 (I believe) and there was already quite a hate crowd surrounding it already even without any real footage, pictures, or even how it would even play! This kind is called ignorance because the Wii had terrible graphics compared to the PS3 and X360, but I don't mind. I played my CoD on my Wii and enjoyed it. Even if I had a PS3, I played a lot more Wii until the games I wanted were all over then I moved to the PS3 more.
Nintendo is a really great company and I've been a long time fan of theirs. I love almost every franchise they make especially Zelda, Pokemon, and Mario.
Now going back to the topic, "haters gonna hate" is basically originated from these common causes. They will be hating and will never stop unless you actually convince them, but that would be as hard as cracking a nut with a finger especially if they are too stubborn to open up to new ideas. They will forever hate until there is something that will expose them to the reality and what is happening around them.
I'm glad I have a community like this to back me up in times of need. You guys are really awesome, and I have never had a fandom like this before. Other fandoms were just plain out gross, annoying, or childish. Thanks for the great times! Let's continue to make more...