Why do fuel tanks of a rocket need to be kept cold before launch?

Many rockets (Including the Space Shuttle) have cryogenic rocket engines. These are rocket engines that very cold liquid propellants. The most common cryogenic engines use Liquid Oxygen (LOX) as an oxidizer and Liquid Hydrogen (LH2) as fuel. Rockets require the fuel and oxidizer to feed into the engines at a very high mass flow rate in order to generate the necessary thrust. The solution is to store oxygen and hydrogen in liquid form, where they are more dense (meaning that they will be fed into the engine at a higher mass flow rate). Because oxygen and hydrogen are gaseous at room temperature, they must be cooled to very low temperatures in order to take on a liquid state. Oxygen must be cooled to -183 degrees C (-297 degrees F) and Hydrogen must be cooled to -253 degrees C (-423 degrees F).