Never make a level without a goal in mind. I must clarify that this is very anecdotal; in fact, many people claim the opposite, that you should just build without thinking about it too much. This has never worked for me. I need any level I make to have an idea, a concept behind it, which motivates me to keep building, if only to carry out the concept to completion. This could be as simple as "a level in XYZ creator's style", but it has to be vivid enough to propel you forward.