Yesterday I was writing about having a sense of purpose, and the "so what?" problem.
We want our readers to be able to see a sense of purpose. The reader has to be able to have some answer to the "so what" problem.
I think, though, that it's even more important for the writer to have a sense of purpose. You should be able to answer to yourself the "so what" question. Better, if you can answer it one at least two levels: 1. scholarly sources or hard data that suggest a question, and 2. an emotional connection to the significance of the work. For example, you can both show that a significant population is affected by some issue, and you feel that the issue is important enough to pursue.
When you are able to answer your own "so what" question, then you can make plans to bring your vision to reality. I was writing about the writer whose committee thought the work frivolous. First off, if you have a good answer to the "so what" question, you're going to know that your project is not frivolous. That in itself is valuable. But with the sense of purpose, you can also begin to make plans. Ok, so your committee thinks the work is frivolous--what can be done to change that? I know that there are conservative, hard-headed, close-minded people out there, but that doesn't mean that they all are. If you have a good sense of purpose, you can develop an argument that matches your sense. Or at least, you can make plans on how to overcome resistance.
Professors do try to exert their influence. Many may even try to set the parameters of the discussion. But--and perhaps I am just giving your average run-of-the-mill professor the benefit of a doubt--most professors who try to give direction do so from the sense that the student doesn't have direction. It's not that they need you to go in the direction they suggest, it's that they need you to go in some specific direction. If you can show them that you have direction, and that you have a justification for your work that can be founded on respectable literature in your field, then you get to set your own agenda, because your professors will see that you have direction.