It has long been known that many massive galaxies in today's Universe show low or vanishing levels of star formation activity. Several promising explanations have been proposed that explain these observations, but a number of theoretical puzzles remain. In particular, the theoretical challenge has deepened with recent observations of passive galaxies at earlier times when the Universe was merely a few billion years old. At such early times, strong gas accretion from the cosmic environment, as predicted by cosmological simulations, should have sustained a vigorous star formation activity. I will discuss recent insights based on ultra-high resolution, cosmological simulations and present evidence that the star formation activity is dictated both by processes operating within galaxies as well as by their cosmological environment. This new perspective aligns well with recent empirical approaches and clarifies the role of internal and external processes in regulating star formation in galaxies.