Observational evidence suggests that disc galaxies are common in the universe, but they can be challenging to reconcile with hierarchical formation models like LCDM. Using the Illustris TNG100 cosmological hydrodynamical simulation, we select a sample of disc-dominated galaxies, i.e. less than ~10% of their mass in a spheroidal component. At a given stellar mass, we find that disc-dominated galaxies form in dark matter haloes that are 2 to 10 times less massive than those of spheroid-dominated galaxies. The most rotationally supported discs are found in haloes with the best-aligned gas angular momentum, while halo spin appears to have no influence on galaxy morphology. Mergers are less common in the formation of disc galaxies compared to spheroidal ones, although there is significant variation, with some disc galaxies having 15%-30% of their stats from accreted material.