There are mainly 3 sources of humidity
- Change of air temperature
- Cooking, taking a shower etc.
Change of air temperature
Warm air can contain more humidity (vapour) than cold air. If the temperature drops humidity condensates. Usually this can be seen first on glass or metal.
You will be familiar of this effect from when you experience ̣"misting" on car windows or your house windows.
You and your pets expel vapour into your environment with every exhalation. You wash, take a shower etc. All these activities produce humidity and saturate the air until the point, where it has to condensate.
A well insulated property doesn't allow the temperature to dramatically. However, a ventilation system removes the humid air out from your house and replaces it with fresh dry air from the outside.