Water exists in 3 states: solid (ice), liquid (water), and gas (water vapour). Water in liquid form is relatively dense, and ice slightly less so. This is largely due to attraction the molecules have to each other. The molecular attracton makies water form droplets on a surface.
Water molecules in liquid form bonds which create a dense material.
In vapour form, water molecules do not have this bonding. Water Vapour is very light compared to other gasses in the atmosphere, which is 78% nitrogen.
The kinetic energy of water molecules in gas form creates space between the molecules.
The high humidity created while showering causes condensation on even relatively warm surfaces because of the high concentration of vapour.
In warm conditions, condensation will form on a cold surface, even when the concentration of water vapour in the atmosphere is low.