The air inside of a tent will condense during the night, just like the air in a car often condenses on the windows. A tent is often not well ventilated in the interest of trapping heat this means that the moisture cannot escape outside. It is not always your breathing that caused the condensation – if you are keeping any wet items inside your tent – especially if you are hanging it up – the evaporation from your clothing or other items is likely to be contributing to the condensation. If you are having a steady stream of moisture from one spot in the tent then it might be worth investigating if you've sprung a leak though.