Some of us might be tempted to think that The Will of God and Pain are the exact same thing. No, God isn't a sadist!
Sometimes doing the will of God requires sacrifice. And yes, in the economy of salvation, to save souls, certain discomforts must be borne to merit the graces necessary. These discomforts include vigils (losing sleep), extended periods of prayer, corporal mortification, fasting, pebbles in shoes, the hairshirt, abstaining from good things, etc.
HOWEVER
If I were to assume that "the more discomfort and pain, the better" then I would be sadly mistaken.
Which would make me happy? Having a happy marriage, or living through a divorce and family breakup post-adultery? Happiness is a weakness and a condescension to selfishness, right? Should I then sabotage my marriage so I don't have too much happiness on earth? Am I being selfish by enjoying a harmonious marriage?
Of course not! A happy marriage is not only preferable to me, but it's God's first choice as well! Even if I wanted to be the greatest of saints, I couldn't do ANY BETTER IN THIS DEPARTMENT than to keep my marriage running smoothly. In fact, I'd be REDUCING my standing in God's eyes if I purposely sabotaged my marriage to offer more "suffering for souls".
Moral of the story: Something good for you, if it's God's will, is not being selfish!
Hmm... I wonder, does this include weekly Mass?
As a corollary, a mother of seven children would be committing actual SIN if she absented herself from the home too much to spend more time in prayer. The Will of God for her is to raise her children and help them become saints. Even something good (hours of prayer) would be a bad thing because it's not in line with her duty of state.
Second Moral: The Will of God is not simplistic or one-size-fits-all. The Will of God is whatever God wills for you personally!
Some parts of His Will are universal (e.g., following the commandments, meriting heaven) but when it comes to duty of state, it depends on the person and his particular circuмstances. That's where all the mystery and uncertainty about God's Will comes in! To determine these more mysterious parts of God's Will (e.g., what is my vocation, where should I go to Mass) we must pray and exercise the virtue of Prudence.