Why does the nacelle always need a powerful cooling system?

Question $12$ options:

Even with a large amount of lubricating fluid, the gear box generates an enormous amount of heat

The direct sunlight shining on the outer surface heats up the interior of the nacelle

The high$-$speed shaft does not use low friction bearings to support its rotation

The generator is not $100\%$ efficient in converting mechanical power to electrical power. Some of the energy is wasted as heat