There're several different types of rivalry. There's rivalries that arise from specific matches or seasons, like Luton, Burnley, Southampton. Then the more traditional geographical rivalries, Bristol teams, Exeter, Torquay. The Pompey rivalry is quite unique as it's a city rivalry as opposed to a geographical one - the two naval ports having a historical rivalry that pre-dated football. Other examples of City rivalries arising for non-footballing historical reasons would be the Yorkshire clubs' collective hatred of the "scab" Nottingham teams. My mate at the age of 11 had a car driven at him almost knocking him down by Sheff UTD fans whilst following Forest, it's an intense rivalry fuelled by long-standing political hatred that's often overlooked.
If you look at Colchester, they'd consider Wycombe their biggest rivals and yet there's no geographical proximity between the two - their rivalry with Wycombe is like a far nastier version of ours with Luton (which has all but vanished now), all arising from events on the pitch during both teams' title challenge in the Conference during the early 90s.
Then you have Geographical rivalries which die over time due to lack of exposure between the teams - e.g. Wycombe and Staines, Stevenage and Enfield. This also applies to us to a lesser extent with Torquay and Exeter. But the team that falls behind the more successful team (e.g. Staines, Enfield, Torquay, Exeter) often builds up a level of disdain (or envy!) for the more successful club. There is no doubt that Exeter's hatred for Argyle peaked when they were a few divisions below us, at a time when our regard for them as "rivals" was probably at its lowest ever ebb.
It's also interesting to note rivalries that only exist because of individual personalities at the club. So Argyle have next to no disdain for Saints now that Rupert Lowe has nothing to do with them, and I imagine now that JFK has long left Luton few fans bare them any malice (beyond the obvious fact that Luton is an EDL-infested hell hole).
So yeah, lots of different dimensions to rivalry!
Another interesting one is politcal rivalries. Rangers/Celtic aside, English football clubs seem to have steered clear of aligning themselves with politics. Foreign leagues are different - you have Livorno, renowned for their communist following, Lazio, renowned for their fascist following, which creates rivalry that has nothing to do with geographical proximity or past events on the pitch.
Also worth mentioning rivalries that cross national boundaries - Chelsea and Barcelona is one that sticks out for me, but I'm sure there'll be better examples. Celtic and Rangers is an obvious example of a religious rivalry, but there're also racial rivalries - in the Israeli league, I think I remember reading that there're clubs with predominantly Arab followings that have fierce rivalries with clubs with predominantly Hebrew followings.
Happy essay writing!
