Comfort, from the dawn of time, has caused humankind to be more lax in its discipline in daily matters. If we are comfortable, why strive, why be diligent in the ways of the Eternal? As we know from a spiritual standpoint, this is a soul-death-wish if we don't attend to the soul on a daily basis. If we have achieved physical comfort, we dupe ourselves into thinking that we have it all. It isn't long before we find ourselves sliding into a pit of despair and spiritual demise because we have ignored the health of the soul.
This story is a challenge to our natures as it applies to this one man. The shochet, or slaughterer, in this story wishes to give up his profession. For him, it is too much of a burden to make sure he fulfills the halachah properly. I once heard a rabbi speak of the laws binding on a shochet. He seemed to imply that they were numerous and made a phonebook look small.
So do we fault the shochet for wanting to give up his profession? If he knows he has such an ability to commit transgressions, wouldn't it be wise to get out of the business and go for something a little more low key? However, as we see from Rabbi Salanter's answer, it is a firm "no". Hopping from one profession to another isn't a guarantee of a life on the beach. The life of a business man is also wrought with issues of honest weights and measures, lying, coveting, defrauding, etc... This is to signify that there are any number of things in which a shopkeeper can be found to be delinquent and guilty.
no easy way out
What this story illustrates is this: anyone looking for an easy way to earn a living, a way in which there are no hard decisions and no stringencies, is living a lie. Life is filled with difficult decisions, attention to detail, and the possibility for mistakes. When we are living an easy life, we become lax and soon find ourselves in the grip of sinful living. For when we have less to challenge our soul and our mind, we fill it with self-absorbing pleasures that are easy to come by. Fruitful living, which requires hard work, diligence and a desire for righteousness, will be the only thing that keeps us from falling into a lifestyle of gluttony, sinful pleasures and utter selfishness. Allow the difficulties and scrupulousness that you must keep up in your life to be a barrier to laziness and laxness, and a call to higher living.