What's your null hypothesis?

You know that one of your friends is reading a book that you lent him late last week, and that he usually returns the books you lend him after about three weeks. Today is Saturday, and you know you yourself typically do most of your reading on the weekends, and that you and your friend have similar schedules and obligations. You thought the book you lent him was a good book, but no more gripping than the typical book you lend your friend, and it's about average length. You read it in three days. What chance should you estimate that he already finished reading the book? What chance should you estimate that he finishes it this week? What chance should you estimate that he reads more than 20 pages today? What chance should you estimate that he'll read this week at all?

Is your null hypothesis that he didn't read today or that he did?

The concept of privileging a null hypothesis seems odd to me if it's not the thing that would be our default assumption to begin with. Things have effects. People and animal do things, and our behaviors are affected by our environments and by the things we eat, etc. (Like the paint that is chipping off the walls, yum!) Why do we privilege the hypothesis that this doesn't happen?

Also, why would we perform experiments without expecting them to have a particular effect? Shouldn't we base our experiments on our beliefs about the world and look to prove or disprove our beliefs based on finding these things or failing to find them?

By the way, I think that the default null hypothesis that people would have for the questions I asked at the beginning are different depending on how the question is phrased and often conflicting. Most people would consider a null hypothesis of reading being evenly distributed over the week as a better null hypothesis than it being concentrated on the weekend. But the default null hypothesis for how long it will take him to finish reading the book would probably be three weeks given the background I gave. But the default null hypothesis for whether he read today would probably be "no" instead of "yes."