I'm a goal-oriented person: I do best when I have something concrete to work towards, and I find deadlines motivating. I often impose structure on myself; even internal accountability helps me work. I've participated in groups where everyone sets weekly goals, and I love that framework. Others may like the sensation of having a cheering section, or be motivated away from the guilt of having to report failure. For me, the motivating part is setting it down in concrete words (in front of witnesses is a bonus; it keeps me honest with myself). I don't always meet my goals, but when I don't, I've made a conscious choice that my priorities need to go elsewhere. Having a specific goal, as opposed to, "as much as possible," actually, for me, makes me feel more free to adjust when necessary.
Also, I just really like numbers. It's a bit of a disease, really. I like tracking and percentages and totals. I like measurements. (This is a good thing for a pastry chef, really.)
I'm not quite ready to add personal writing goals to my plate right now; I have a lot of balls in the air presently. But what I am going to start doing is tracking, so I can get a feel for how much I'm getting done per day (and probably be highly embarrassed by it). That may also give me an idea of what a reasonable weekly goal might be ...
See what I mean? I'm setting goals for the purpose of figuring out what goals I should set.