Why should or shouldn't the greenbelt be built on?

We shouldn't build on greenbelt because it is more lovely to look at rather than to actually occupy with buildings. It may create more homes but the world and it's natural objects should be preserved for future generations and should be looked after and cared for by this generation in the hope that we are doing the right things to the natural world in some places rather than what we are doing in certain area. e.g climate change etc.