Green is a colour of nature, and is the colour that humans can see more variations of than any other. Because it is associated with plant life and the growth thereof, green is often association with springtime, freshness, and hope and is often used to symbolize rebirth, renewal and immortality. It is also associated with environmental improvements, with environmentally sustainable or friendly groups or products claiming to be 'green'. As a cool colour, it reminds of cool shade offered by trees.
There is a saying, you may have heard that says that 'blue and green should not be seen without a colour in between'. Of course, nature renders this saying complete nonsense as we often see green trees against a blue sky - a sight which for most of us is pleasant and positive.
Green does have some negative connotations, the best known one being its association with envy. It is also associated with unpleasant textures such as slime and is said to be the tinge that our skin acquires when we are ill. It is also the colour of clouds when they are laden with hail.
Combined with colours close to it on the colour wheel, greens exude a feeling of peace and calm, whereas when combined with its opposite, or complementary colour (red), it creates a strong energy within a composition. See Colour Blog #3 for more about harmonious and complementary colours as well as red/green colour-blindness.