As you said love, also called charity, is indeed the greatest virtue, and St. Thomas Aquinas even says that no true virtue is possible without charity , and shows this by referencing (1 Corinthians 13:3): “If I should distribute all my goods to the poor, and if I should deliver my body to be burned, and have not charity, it profiteth me nothing.”. Now that we know that charity is the greatest virtue we have to know precisely what charity is. The Catechism of the Catholic Church (1822) says that “Charity is the theological virtue by which we love God above all things for his own sake, and our neighbor as ourselves for the love of God.”. All true charity is directed towards God, and Jesus himself, when asked what the greatest commandment is, said “‘Love the Lord your God with all your heart and with all your soul and with all your mind. This is the first and greatest commandment."(Matthew 22:37-38). So if we truly have the virtue of charity we will love God above all other things, and if we love God above all things we will wish to keep his commandments as Jesus said “If you love Me, you will keep My commandments.”(John 14:15). To answer your question about why fasting is important, fasting allows us an opportunity to show our love for God, and become like him by uniting our sufferings with those of Jesus on the cross. It is important to go to mass because the mass is God’s greatest gift to mankind, it is through mass that the merit of Christ’s death on the cross is applied to us, and in the mass that He comes down to us in the form of bread and wine as a sign of his infinite love for us. Imagine that you were homeless and in terrible poverty and another person put themselves into large debt to buy you a house, and you decided not to live in the house, and to never speak to the person who bought you the house. This is how it is with people who don’t pray or go to mass, Jesus died a painful death on the cross for us so that we may receive Him in the Eucharist, receive forgiveness of our sins in confession, be baptized in the name of the Father, Son, and Holy Spirit, and so that when we pray we may call God our Father.