Reasons for the Sunday Mass Obligation
“On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body.” (CCC 2185)
It seems to me that most serious Catholics are not going to Mass every Sunday in abject fear of mortal sin or losing their salvation (i.e., as their primary or main reason), but rather, because they love it; they want to go, and understand the joy and necessity of regular assembly with other believers and receiving Our Lord in the Holy Eucharist, which gives us all sorts of power and sustenance to face the trials and challenges of life.
We can all fall into legalism (or the opposite error of license and antinomianism): being human and sinful. But as we advance in the Christian life we have to progress beyond that. The Mass obligation is not about rules, but about “spiritual normalcy” or “spiritual bare minimum obligations.” The Christian needs to worship with fellow believers, as a good and helpful thing:
Hebrews 10:24-25 (RSV) and let us consider how to stir up one another to love and good works,  not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near. (“classic” KJV rendering: “Not forsaking the assembling of ourselves together”)
Acts 20:7 On the first day of the week, when we were gathered together to break bread, . . .
Acts 12:12 When he realized this, he went to the house of Mary, the mother of John whose other name was Mark, where many were gathered together and were praying.
Acts 13:44 The next sabbath almost the whole city gathered together to hear the word of God.
Acts 14:27 And when they arrived, they gathered the church together and declared all that God had done with them, and how he had opened a door of faith to the Gentiles. (cf. 15:6, 30)
Psalm 102:22 when peoples gather together, and kingdoms, to worship the LORD.
It’s fundamental to Christianity to gather together to worship the Lord, and to engage in liturgy, rite, and ritual. It’s plain in Scripture that this is a good and worthy and spiritually helpful thing.
All the Church is saying is that it is so important that it should be made obligatory for the Catholic who has voluntarily submitted to the authority of the Church. We make things mandatory because of their importance.
The obligation is primarily intended for those who are weak, because human nature requires rules before it can properly understand a willing and voluntary spirit, because something is good to do. That’s why we have lots of rules for children, because they are too young to make wise choices. The older they get, the less rules are necessary, or else they “self-supervise.”
Too many Christians are not motivated out of sheer love for the Lord and desire to please Him and to lead a saintly life. This lamentable deficiency is likely true at least some of the time for everyone, no matter how pious or devout.
The Church in her wisdom makes something compulsory, lest the human tendency to laxity cause many to not attend church. Is that a good thing all in all or a bad one? Is it a “net gain”?
Of course it is good. It’s better to have someone be in Church, even though they are not perfectly motivated from the heart and soul, than not to be there, and sitting at home watching the Sunday morning news shows.
The ideal of the Christian life is wholehearted service to God and completely pure motivation: doing everything for the right reasons, by God’s grace. But the Church exercises wisdom in requiring church attendance, for the sake of those Christians who are merely “coasting” in their spiritual life, going through the motions without much heart or interior motivation.
Both are good. We all should strive for the ideal, and pray for God’s grace to achieve it, but we should also be glad that many a “Joe Q. Catholic” is in the pews even though he is there because he “has” to be there, not because he wants to be.
God in His mercy accepts millions of His followers (at the “spiritual milk” stage) as they are, warts and all.
In conclusion, the Catechism of the Catholic Church has lots of great insight about Sunday worship and rest:
2182 Participation in the communal celebration of the Sunday Eucharist is a testimony of belonging and of being faithful to Christ and to his Church. The faithful give witness by this to their communion in faith and charity. Together they testify to God’s holiness and their hope of salvation. They strengthen one another under the guidance of the Holy Spirit.
2183 “If because of lack of a sacred minister or for other grave cause participation in the celebration of the Eucharist is impossible, it is specially recommended that the faithful take part in the Liturgy of the Word if it is celebrated in the parish church or in another sacred place according to the prescriptions of the diocesan bishop, or engage in prayer for an appropriate amount of time personally or in a family or, as occasion offers, in groups of families.”
A day of grace and rest from work
2184 Just as God “rested on the seventh day from all his work which he had done,” human life has a rhythm of work and rest. The institution of the Lord’s Day helps everyone enjoy adequate rest and leisure to cultivate their familial, cultural, social, and religious lives.
2185 On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body. Family needs or important social service can legitimately excuse from the obligation of Sunday rest. The faithful should see to it that legitimate excuses do not lead to habits prejudicial to religion, family life, and health. ..