From the chapter "Vocation" in Thomas Merton's No Man is an Island:
"The fulfillment of every individual vocation demands not only the renouncement of what is evil in itself, but also of all the precise goods that are not willed for us by God."
I really appreciate Merton's insistence in this chapter that we simply can't have it all. I spent quite a bit of time trying to get this across to my students last semester when we were reading Annie Dillard's The Writing Life: excellence demands commitment which in turn demands sacrifice. If you didn't want to do it, or it was sin to begin with, where's the sacrifice?
Yes, yes, it can feel like sacrifice to give up sin, but ultimately it's not sacrifice in the same sense that I am meaning here -- of course I should give up sin! (I don't mean that the struggle isn't a kind of sacrifice.) And sin, while it has its pleasure for a season, turns to ash, so why should I, in the end, consider it sacrifice to give up what will only obviously harm me?
The kind of sacrifice Merton means here is giving up what is in itself a good for the sake of one's calling. There is nothing wrong with many activities -- except that they may be inappropriate for me and at this time because, while I won't be "sinning" in pursuing them, my vocation simply doesn't leave the time and space for them. Thus, if I do pursue them, I will fail to fulfill that which God has called me to do and be. I might not be sinning per se -- but I will be something other than, less than, what God created and intended me to be.
For a student, this may mean something as simple as giving up a movie or fellowship to study for an exam or write a paper. But to fulfill a life-long vocation requires such sacrifices at a much greater level -- obvious examples that spring to mind are the sacrifices of personal freedom and family time made by physicians and pastors and those in the military or law enforcement.
But whatever the call is, there will be sacrifices. We simply cannot have it all. And one of the keys to contentment is accepting this by embracing God in the call itself and loving Him by our service to Him because of His love for us.