Part of the allure of buying a home is stability -- the same neighborhood, the same schools, a predictable payment. Although the location of your home won't change, the character of the neighborhood can, school districts can be redrawn, and your monthly mortgage payment can go up (or down, but up is more common).
If you have a fixed-rate mortgage, it might seem like your payment would stay the same. A stable payment might even be one reason you decided to buy rather than rent. So, when you get a notice that your payment is going up, what could be the reason?