Why does it rain so much in the tropical rainforest?
Rainforests tend to be near the equator.
Part of the rainfall is that it's just near the equator, so temperatures and available water are higher. Then it's really that the rainforests are there because of the rain, not the other way around. But rainforests actually do cause rain to a degree.
All plants undergo transpiration, where they take water in their roots, it goes through their system and then leaves through their leaves (or leaf equivalents) as water vapor. Since tropical rainforests have so many plants, there's a ton of transpiration. When you get that much water vapor hovering over rainforests, it's bound to rain a lot.
The phenomenon can actually be observed as people are cutting down the Amazon. The areas that are getting cut down are actually getting decreased rainfall and humidity.
In sum, tropical rainforests only exist in areas of high rainfall, but they also cause more precipitation through transpiration.