Magnetic fields are ubiquitous in the Universe. The Sun’s magnetic field drives the solar wind and causes solar flares and other energetic surface phenomena that profoundly affect space weather here on Earth. The first magnetic field in a star other than the Sun was detected in 1947 in the star 78 Vir. Today, we know that about 10% of these intermediate-mass and high-mass stars have strong, large-scale surface magnetic fields whose origin has remained a major mystery till today. In this talk, I will present the first 3D magneto-hydrodynamical simulations of the coalescence of two massive main-sequence stars and 1D stellar evolution computations of the subsequent evolution of the merger product that can explain the origin of strong magnetic fields in massive stars. I will argue that such magnetic massive stars are promising progenitors of those neutron stars that host the strongest magnetic fields in the Universe, so-called magnetars, and that may give rise to some of the enigmatic fast radio bursts.