few if any people on the planet have ever had the capacity to generate their own emotions without having stimuli to start them. possibly the exception to this is with meditation, but even here, there is a type of trigger for the contentment that sometimes arises out of meditation. the trigger is the release from the usual traffic of white noise/babble that fills our minds often.
but otherwise emotions arise independent of our direct choosing if we could by simple willpower generate the emotions we wanted we would be able to be permanently content by simply willing it... clearly we can not do this we need stimuli to generate the emotions that we want and then we hope that these emotions arise. when negative emotions arise they are provoked by a stimulus and we do not choose them... ...they choose us thus emotions are autonomous programmes
it can be seen that they are linked to survival of dna given that some of our strongest emotions are the urges of love and sex which obviously promote the survival of dna