I'll go out on a limb and say it would be pretty rare to find a lifelong meat-eater working for PETA. It's fair to say that if you work for, volunteer for or donate to an organization, you pretty much would and should love it and everything it stands for. A fundraiser's job is to create compelling campaigns that tap in to that love (or in some cases, remind the potential supporter that he or she really does have a soft spot for, say, war orphans in Africa).
So fundraising is all about the mission. Well, duh. It's also about proper and timely asks targeted to the right audience and followed promptly with a thank-you note, receipt and another ask. But no matter how well you know the fundamentals of fundraising, if the mission isn't something people can get behind, then they simply won't get behind it. That's not to say every mission has to be happy and giggly and swarming with rainbows and unicorns. There are "unpopular" missions that don't paint pretty pictures or lend themselves to cute art or flashy websites. But they're no less worthy than those that do.
Your job as a fundraiser is to know your mission inside and out. You have to know not only who your potential supporters are, but why they give and why they give to you. You have to be aware of the nuances of your specific mission and the people who support it. For example, one of the first and more fascinating glimpses I have gotten into the donor mind-set came years ago, when someone told me that if you're talking to potential donors about their pets, for Pete's sake don't refer to a pet as an "it." That donor's pet is a he or a she or, ideally, a Boscoe, Fluffy or Scooter. Loving a pet means acknowledging that he or she is just a little person with fur. And courting a potential donor to an animal-welfare cause means honoring that acknowledgment.