Hello everyone, Mr.Gol here. I am here to explain or, in this case, give my theory of why we hate our rivals. Now everyone might say "This guy is an idiot" but let me explain. One day I was just sitting down on the bleachers of my school (in the handegg field) and I was wondering why we hate, and I mean hate, our rivals so much that we would beat them up, say means comments or kill them. Okay not all fans are like that but some are. Why? Well let me put some examples

• Influence
  You know some kids would grow up like their father or mother. And we got some traits from them. For example, from my mom, I am very nice to everyone and from father, well, I get very pissed when my team loses. Well my dad barely watches football but back then when he watched football, he always got pissed whenever Club America, that is the reason why I like Club America because of my dad, loses a match. Also he cusses, not in front of their face, whenever he sees a chivas, UNAM, or Cruz Azul fan. I also get pissed whenever I see a rival fan. Coincidence? I DON'T THINK SO!!!

• Passion
  You know those people that are really passionate with their club. Well, I am one of them. I can't stop watching football. But I am not one of those fans that beat up other fans. Because I am weak. XD. The passion drives in our blood and we want to win. We love our clubs and we would defend the club at all cost. Okay not all fans beat up people but we do have passion. When we finally see our club play after a week or more and then lose, we take our anger on others. No one likes to lose but we must accept. Some club america fans are like that, and I don't agree with them.

• Differences
  It's true that everyone is different and there is no problem with that. It is okay to be yourself. But in the football world, it is kind of different. Let's use me as an example, I was a sophomore, I am walking to biology class and I sit down on my seat. I meet this kid that likes football. We did't talk much but in the next day when I wore my club america shirt, he hates me. He was a monterrey fan but still hates me. And then when a new kid of the class (not the school, just switch class) sees my shirt and hates me. WTF? Well, Club America is the most hated club in Mexico and he was a chivas fan. This is why I hate Biology. These kids annoys me the whole time I was in Biology. Until the Monterrey fan and I was kind of fans but not much. The china fan. I will always remember his name and how he looks. Also, some rivalries have one team represent the working class and the other the "rich" class. Other rivalries that the teams are different due to religious belief, or political belief. Like Real Madrid and Barcelona, Inter Milan and AC Milan, and Lazio and Roma. We all have different beliefs and that influence our choice of theca if we live in the city with two teams.

Last but not least. Drum roll please. Drum Rolls

• We have to or just because
  Yep, this is the last reason. We as fans have to hate the rivals. Well some new fans see that the club they choose are rivals with one club. For example, Me when I realize I see that Man U are I rivals. I hate them from that point on. Well, I do dislike them, I respect them as a club. Well, this isn't to the crazy fans, this is to all fans. Once we realize we are rivals, we hate them. Just like that.

Even though some fans do damages, others fans just want watch football. They just want to get a beer and drink in peace. Thank you everyone for reading this. I wrote this to express my opinion and so that you can read this at your toilet. Thank you and have a nice day or night and have a lovely pooping

i would separate the cause of rivalries into three groups: geographical, success and personal.

geographical: two teams form the same city are bound to dislike each other, e.g. manchester united and manchester city
success: teams that are successful and compete for the same trophies dislike each other, as each team wants to win that specific trophy, eg. barca and madrid. moreover, the more successful a team is, the more envy it generates from supporters of less successful teams.
personal: personal experiences can influence your like/dislike of a club. for example, i dislike chelsea because of my brother, who is an avid chelsea supporter.

I don't hate United's rivals. I actually have a very healthy respect for Liverpool, and even City.

Maybe I'm alone in that perspective, but I don't think so.

@Sunflash No, but when someone beat your team, you know you're pissed. :P

If it wasn't for the retarded fans then i wouldn't hate any team, even arsenal and tottenham.

@Emo

Usually I'm pissed more at United than whomever beat us tbh. That was certainly true all of last season.

@Sun What about international game where your team get beat by germany? ;p

More mad at Canada's missed chanced than anything else. We gave away a PK for no reason at all, I'm sure you remember that :P

To be sincere, I didn't watch that game, I was more mad against Canada in the 2015 world cup where we had a pen and managed not to score it in the knockout face. :(

I'd like to add Political to @nanda's list. Real - Barca are an example of a political rivalry. Dortmund - Bayern are a better example of a success-based rivalry.

@Dynast Too true, lad, too true.

@Dynastian I actually put that in difference section.