The very best startup ideas tend to have three things in common, Paul Graham says:
They’re something the founders themselves want, that they themselves can build, and that few others realize are worth doing. Microsoft, Apple, Yahoo, Google, and Facebook all began this way.
The verb you want to be using with respect to startup ideas is not “think up” but “notice.” At YC we call ideas that grow naturally out of the founders’ own experiences “organic” startup ideas. The most successful startups almost all begin this way.