Someone might just annoy you, you might just completely disagree with they points of views, they might act like they are superior, they might show off, maybe act like they are cleverer than they are, it might be parental or media views that you agree with.
The simplest way is just to dislike everyone & everything, then you can be surprised when something happens & change your mind & no longer dislike someone or something, thats a better way than liking everyone & everything, then being let down or annoyed for no reason

Better wait for the queueing file, might be a red snapper...

its because when people post questions and ask for opinions they cant handle differant opinions to themselves if they get one they act all high and mighty and argue back rather than respect others opinions and then after all that they rate your answer unhelpfull. lol

I am not a hoody, I am a parrot.

This is a hoody

I think the saying about First Impressions couldn't be truer. And in Jade Goody's case, everyone's second impression was even worst than their first.

It's our old friend the genetic imperative's fault.

When there was not much time for romance and you had to plant your seed and get out, we needed to form instant assessments of our fellow humans.

This followed for the potential opponent as well.

We still have the vestiges of this instinctive ability - we've all probably formed an instant dislike for one person within a group, without knowing the reason.

With specific reference to the point about it happening on here - communication is only about 2% verbal with the other 98 coming from body language, inflections etc. In print we only have the 2% and misunderstandings are so easy it's almost unbelievable.

It happening here I think has to be like what Hiheels said in the sense we only have the writen word and misunderstandings are very likely to happen. Some people are more likely to question back and say something about the comment given the person to clarify themselves and sort it out but others can take one look at a comment and form an opinion and go away with it. Often in the case of misunderstandings communication from both parties immediately can resolve things otherwise I think damage is done.
In a general sense sometimes you might have caught that person at a bad time or you yourself was feeling grouchy when you formed those impressions (im talking of a negative impression here obviously a more positive outlook might make you form a positive first impression.)
Sometimes and overall nice person might have made a comment on something that you are particularly sensitive to which causes an instant negative response.
But there are a few times when I do take an instant dislike to someone without know why at all... I know I dislike the person but I cant put my finger on why... I will have to go with Candys genetic theory for these cases perhaps we are just wired that way to have a instant like/dislike response at times.

I most cases, I don't think I do have instant dislikes - it is just that it doesn't take me long to form an opinion of someone. On this site, people rating questions really badly or being rude for no obvious reasons make me look into their other questions, and based on a combination of things, I will decide whether it was a momentary blip, or whether actually, I will never be able to reconcile my views with that person.

There are some users who I violently disagree with on some issues, but that doesn't mean that I take an instant dislike to them - often, on other issues, you might agree with them.