There's a document that I think was an unofficial project management document from NASA, I can't find it now, but one of the items was something along the lines of 'if your space mission isn't reusing an existing launch vehicle, it will be a launch vehicle development project and everything else, including what you think is the focus of your mission, will be secondary to this'.

Edit: additional bit I remembered, this statement is prefixed with something like 'you may start thinking you project will be cheaper/more efficient if you develop a launch vehicle suited to your particular mission instead of compromising on an existing one, but', then the above.