Once you gain control of a place, live there for numerous generations, create a large amount of history, it begins to be viewed as "yours". Same way that my fellow Americans and I transformed a continent with a rather large native population into a land where they're a very small minority in our vast society, and how this is viewed as "our land" now.
Possession, by whatever means, leads to ownership of territory. It's the same reason why Mexicans still tend to harbor a grudge against the US due to the Mexican Cession that ended the Mexican-American War, where we took about 1/3 of their country. That land, now the states of Arizona, New Mexico, Utah, Colorado, California, plus additional territory, are viewed by Americans as a part of our homeland, just as Mexicans see it as territory that is their own but was stolen from them.
Until the population that harbors the sentiments vanishes or otherwise forgets, they'll always want to reclaim what they see as "theirs". For similar examples around the world, see the islands north of Hokkaido, Japan that Russia owns, or Taiwan/China, or Palestine/Israel, or Russia and the Baltic States, etc.
From the EVE perspective, you see this in Amarrian glances towards Matari territory that they were forced out of, or Caldari Prime from both the (current) Gallente and (prior) Caldari sides. People don't like putting down roots, and then getting evicted, they'll want action to take territory back - if it doesn't happen in a few generations though, things might settle into status quo, the whole "it's always been like this for us" point of view.