Do you lose interesting in CS when you have app/game ideas of your own?
Maybe when you don't have your own app/game ideas, studying CS is the only thing you can think of doing?
From the viewpoint of CS you can focus your complete attention on something very detailed, such as how to build a Suffix Tree in O(N log N) and what feats you can accomplish with it, things that are not suitable at all.
From the viewpoint of developing an application though that is just a means to an end.
I think I'm constantly pulled by CS fundamentals. I read it from some book about the development of Windows NT that "...he is more interested in computer itaelf than applications". I do have a hobby gamedev project but it never beats computer itself.