I grew up poor. Pretty close to poverty-level poor. I received free lunches in public school and that sort of thing. Pure suck. People in this country treat you like you have a communicable disease when you are poor.
As soon as I was able to, out of necessity, I started working part-time jobs to help my family out. Paper-routes, dishwashing, and retail. (Believe it or not, I preferred washing dishes to retail, where you have to deal with sleepwalking consumer morons all day as they purchase things they don't need and treat you as though you're a neanderthal just because you're working for Big-Box-Store-X. At least when you're washing dishes, it's just you, a basin of scalding water, and dirt. I can handle that.)
It wasn't long before I started to resent my employers. It doesn't matter who they are. They're all employing you because they're making more from your efforts than they're paying you. I have a great work ethic and never let the quality of work suffer due to this realization, but the fact remains, and it's tough to deal with.
Some history and background: I went to undergrad college thanks to having generally good teachers for whom I earned great good grades in HS. One in particular urged me to apply to good schools. Plus I received generous grants from the government and my university (Boston College) because of demonstrated need. Got a degree in computer science. Have been working 14 straight years since, with little vacation. Saved a lot of money. Still, I've resented the incredible workload and levels of stress that are inherent to IT and software development/support. Many of the jobs I've held are 50+ hours a week and you definitely take the work home with you in the form of a) 24/7 on-call duty and b) not being able to shake the thought of the mountains of work you'll shortly be returning to after your day or two away from the office. It's also common to be faced with technical problems which require creative troubleshooting and resolution, which tends to happen on weekends or in your sleep as you dream in scripting/code. It takes over your life.
So I completely realize that I'm very lucky to be in a good financial position nowadays, and to get there through employment that hasn't broken me physically the way that some stuff does (farming jobs, landscaping-type stuff, etc) but at the time time, I resent having essentially done nothing but work for other people for so many years. And not just work, but work of the same general type, flavor, and unvaryingly crappy consistency. Humans aren't insects, born to perform a single type of task forever, until the day they die; we need to do other things in order to be well-rounded and happy.
For me, I'm motivated to work toward FI to
1. Point to a specific achievement which allows me to conclusively assert that I'm not poor -- I've been cured. I've not only climbed out of the pit into which I was born, but continued climbing onto a pile of money which allows me the freedom to do whatever I please.
2. Stop helping the rich get richer, off of my blood, sweat, and blood.
3. Start doing more with my life than work on a computer all the time. Give back to the community. Spend more time with my family. Get involved in hobbies. Feel more connected to the world, my friends, myself. This is pretty close to the "being a better person" motivation that darkelenchus mentioned.
4. Say to other SUV-driving, McMansion owning slaves that I work with: Goodbye. Have a nice life working to pay for all that stuff you don't need. (This seems to be a common theme in the other posts -- glad to see I'm not alone here. It sounds like I'm being petty when I think it to myself, but perfectly reasonable when I read it from others :) )
5. Helping the unemployment rate go down. When I leave, I suppose my position will be opening up for someone else to take. Just doing my part.
