[Captain1nsaneo]

Why is there faction loyalty? That's an interesting question. A lot of the posts in here nail the core concepts of why. Locked into what ever side you choose, common purpose, and your allies. While it's been mentioned I think there's another thing that hasn't gotten as much credit as it deserves.
The places in Planetside weren't digital. Oh they were actually digital, that's true, but they didn't feel it. It felt like the world was actually a real place. It wasn't done through nifty environments, flora, fauna, or weather. It was done through persistence and that as the war moved so did the land under your feet. You got to know places, learned about them from attacking, from defending. In your head you would get to know the terrain, think about it, where does the ground give me an advantage, where are there likely mines, is that StoneRhino in a liberator over there? Eventually it becomes like an favorite hangout, some continents more than others, but they gain a real sense of physical space over time. This leads to the situation that other games try get with the 'it's our base!' motivation for defense. But in those games that base doesn't have any presence, it's not real. In Planetside, the bases are real.