Greenhouse gases are not all bad. With 90,000 out of every 100,000 years in the planet's history being ice ages, greenhouse gases are absolutely necessary for maintaining the climate we enjoy.
In the absence of greenhouse gases like water vapor, carbon dioxide, methane, etc, the average temperature on earth would be -18°C - pretty darn cold and basically unable to sustain life. However, there can be too much of a good thing.
The concentration and composition of greenhouse gases in the atmosphere has fluctuated throughout history but has been climbing more recently due to human activity - namely, there are three times as many of us as 100 years ago and that results in more methane from us, more fossil fuel combustion, more methane from livestock and various gases due to development of agriculture to feed an increased population.