Why do we hate - for what reason is this emotion present within us? Without having done any research on the subject, it seems to me that it exists as a means of cultural solidarity ("us" versus "them"), and thus serves as a fuel for war.
War seems to be an extension of basic territorial impulses. As I recall from my nature studies (and this seems logical), territories are essentially established based on need and availability. Territorial predators seem to have larger territory radii if they need to hunt more frequently, either because they're larger animals or because hunting conditions are poor. So the "war" instinct to hate those that are different is an instinct to insure group (and thus individual) survival.
Many argue that war, or even violence in general should be abhorred. This too makes sense as an instinct towards cultural cohesion, as group survival depends on cooperation and a highly effect group works best when there is minimal chaos in the system. At the very least, this attitude applies pretty globally to those within the group that the individual identifies as being a part of his family or close companions. Those inclined towards violence towards family members seems to be slightly more rare, although the level of acceptable violence differs by culture and in some cases the violence towards companions may be more taboo than towards family. Violence towards women is portrayed almost casually in some time periods, though not having been that particularly interested before I can't provide an exactly example.
The group survival depends on achieving a state of non-aggression within the group and protecting it from outside interference. The war instinct is needed to wipe out threats or to take the territory of others, and insures healthy competition between different genetic groups which promotes healthy evolution. It can be extrapolated that violence in general serves to this purpose, although often though the lack of violence in aforementioned social circles.
Disliking, however passive, is another form of hate: Aversion to an outside thing or idea. If war is about survival and genetic diversity, then "war" among ideas or things is about survival of the things and ideas, which in turn promote survival fitness in the form of advances in technology and entertainment.
We don't want to interact with something we dislike. If it's a show, we stop watching it. If it's music, we stop playing it. If it's an idea, we don't want to hear it and try not to think about it. When a something disliked strongly enough is liked by someone else, we usually immediately start to dislike the other person unless we actively make an effort not to (and often, even then).
We spend time on disliking or hating others because we are being pushed to be the winner in a survival of the opinions contest, which are founded in the first place essentially for survival. Much like violence, there are many who push for tolerance and understanding of others, which seems to be the defense mechanism against the hate instinct.
In the modern world, hate and tolerance get incredibly complicated. Ultimately, it may come down that the survival balance is frequently skewed in favor of one or the other. If hate and tolerance were of exactly equal proportion, it seems that one would inexorably begin to hold sway over the other, especially as they get woven into taboo and custom. Eventually it would begin to swing the other way around again.
It may seem at first that a society full of tolerance would certainly be preferable, and indeed it may. However, it seems tolerance can lead to inactivity, and is probably linked to personality traits that would lead to a sedate, stagnant life of complacent bliss, such as the one depicted in the movie Wall-E. Conversely, it could blossom into a society of total community and sharing, which has its own issues. A hate dominated society could be Orwellian or it could be Dothraki: The height of oppression or the height of violence, shades away from anarchy.
Interestingly, all extremes yield similar results - the range of ideas and the rate of development is stunted. It seems that balance would be preferred, yet as I've already remarked balance seems unlikely to remain stable for long. Of course, there are many other elements of society in place, and it would be extremely difficult to definitively state what traits and behaviors are truly healthy and which are truly detrimental to cultures as a whole, and is probably relative to the current situation.
So if ultimately both tolerance and hate are necessary to survival as a species, where does that put a person with morality? Is it then morally permissible to hate as much as you want, since it's natural and serves a purpose? Should you be tolerant of everything for the same reason? In some cases both are mutually exclusive sans insanity, so obviously some moderation is required.
My personal take on it is that tolerance should be fostered firstly and foremost among closest community members - family, friends, coworkers and neighbors. Hate or dislike of anything is to be avoided, and if it exists it should be addressed. Obviously, that's not always possible.
In general, I try to check my "dislike" until I am fully informed and have checked my facts; while I may not try to deny the feeling, I will at least be certain I am disliking for honest and informed reasons. If someone has severely wronged you and their involvement in other areas of your life would be detrimental to your continued, healthy existence, then it's permissible to hate them, too, although it may not be healthy to harbor those feelings for an extensive period of time. That should scale appropriately - I will not even dislike someone simply for wearing pink, even though I dislike the color. I will not hate someone simply because they criticized my music, although it may make me like them less for not having similar tastes.
And ultimately, I will try to consider other points of views on some things in an effort to expand my own.
_______________
Again, these are simply my observations and I have not done any research on this subject so please don't repeat anything I've said as fact, though I would definitely love to hear thoughts on my ideas. I intend to later follow this up with some reading into actual research and see how that matches up, and opinions of others would be the first step.