Stimulus in Latin means "sting". Therefore, a stimulus is something that makes you jump, like the sting of a wasp. It makes you jump positively, with joy, like the triumph in a competition or a good joke, or it makes you jump negatively, like the loss of a loved one or an unexpected defeat. Either you stimulate (and direct your own life) or simply react (and you lead the life dictated by others, be they parents, politicians, bosses, etc.).
Do you know what moves you, what makes you jump?