in my last charity job we had to call people up and try and get them to raise more money (not through DD, i might add). I hated doing that, it felt like i was a salesman.
One of the hardest parts of charity fundraising (from the charity's point of view) is getting people, so once people are 'in' it's easier to try and get them to give more - or take part in other money raising campaigns/events than recruit more givers.
some people do feel very passionately about 'their' charity of choice - often because they have been helped by that charity in some way. getting people into supporting another charity is a thankless task at times (from a fundraiser's perspective).
saying that - i'm still looking for another job in the charity sector.