happiness comes at the satisfaction of any stimulas hunger.... not just a food hunger, but any particular hunger that may arise... it lets us know that we have accomplished whatever our mind thought we needed to at the time... which is a key, what we THINK we need is often far more a factor than what we do need.
we cause a lot of un-necessary stimulas hungers when we compare the reality of the life we lead to one we have come to believe we were cheated out of or let slip away or somehow could have had but somehow did not....instead of simply accepting what did happen as it did happen.
but once satisfied, a new hunger is quickly discovered and so it begins again...and there are only rare moments when we truely feel satisfied for any length of time...usually thanks to a liberal application of opiates called endorphins... a feeling of euphoria and satisfaction that tends to last only a few moments...but is so powerful that its triggering accounts for much of what drug users and others who violate laws and morals do.
so addictive that when given a means of self-stimulation via the tapping of a metal bar, mice have been known to starve to death.
understanding that this elusive goal of stimulas hunger is often only a pill or snort away has lead many to abandon normal lives and pursue the "easy" way...which is often no so easy and frought with its own terrors....such as going to prison or being killed in a bad drug deal etc.