As you work in Revit you get used to seeing temporary dimensions appearing every time you select an element. Then as your project progresses and more and more things fill your screen you eventually notice that those temporary dimensions aren't showing up as much. At first you might think that something has gone terribly wrong. The truth is that Revit has reached a point where it has decided that showing temporary dimensions would negatively affect performance.
So now what? Do you have to add your own permanent dimensions to manipulate your design? Well...you could, nothing wrong with that but you don't HAVE to... Enter the Options Bar, when this occurs take a quick look on the Options Bar and click the nice Activate Dimensions button.
Why doesn't Revit tell you it isn't going to display the temporary dimensions anymore? Well...we probably would get annoyed with that message popping up all the time too?