First - the Indian Ocean monsoons (also "summer monsoon" or 'southwest monsoon") occur louder and larger than anywhere else because Asia is a huge land mass in the Northern Hemisphere. Why do you care? Read on.
Well, there are actually three different monsoons of the Indian Ocean and Southeast Asia region, but they all work basically the same way. It has to do with differential heating of land and sea during the summer seasons. Water has a better heat capacity than land (much better), so it absorbs a lot of heat, and not heating up much, while the land is radiating heat into the air, making the land itself very hot, and the air mass above it very hot. As the water gets warmer, it starts to mix vertically, which creates turbulence, which allows more moisture to be released into the air than normal.
As we know, hot air rises, and this is a huge chunk of hot air. As it rises, it creates a dry low-pressure zone over land, which create a strong, stead wind blowing inland (rising air = low pressure zone = air moves in). Now the moist air over the turbulent oceans is sucked in over land. There are a number of different ways, dependent on local conditions, that will make it rain. But as a generalization, as the moist ocean air blows inland, it also heats up, and rises. The rising air cools by expansion in lower pressure higher in the atmosphere, which makes it rain. Since the rain is going to keep up as long as those winds blow and rise in the low pressure zones, it's going to rain for a very long time, and fairly steadily the whole time.
In the winter, this process is reversed. The water retains more heat than the land, which creates high pressure over land, then a breeze out over the ocean.