Between the quotes of Thomas Aquinas this one synthesize, summarize: "We sin or live righteously". The man as a spiritual being, as a creature of God, has to make a basic choice.
There are similar statements in the Bible: "No one can serve two masters, for either he will hate the one and love the other, you can not serve God and money", Matthew 6:24; "For the wages of sin is death, but the gift of God is eternal life", Romans 6:23;  "For those who live according to  the flesh set their minds on the things of the flesh, but those who live according to the spirit , set their minds  on the things of the spirit" Romans 8:5
We have to choose between two lifestyles: the blessed Life and its opposite the unfortunated life.The Gospel of Luke in the Sermon on the Mount tells us about 4 blessings: Luke 6: 20-23 and 4 curses Luke 6: 24-26, these last ones constitute the false happiness. Hence the difference between the ultimate ends of life: the fruits of the spirit (Galatians 5: 22-23) and the works of the flesh (Galatians 5: 19-21).
Basically there are two discernments, one inclined towards good and the other toward evil.
The blessed life or a life full of sins, this is the message of Thomas Aquinas, father of the scholastics. The blessed life is to act always by ethical principles: the golden and the silver rule; the permanent searching for the excellence, to search the correct and the best.
When we are unfortunated people life is rule by the seven cardinal sins: envy,  pride, wrath, greed, lust, gluttony and sloth. The origine of unfortunated life is the exesive interest in the personal matters, happiness at any cost. It is base in our egoism, which is placed over all the ethical rules.
The origine of the blessed life instead is the desire for the good, the desire of improvement, the taste for wisdom. Through it we developed a progressive mentality that allow us to evolve and develop a higher level of spiritual perfection.
The desire for the good is the origine of many skills for example the sacred listening and the third eye: the eye of the mind.
The law of  return: “you rip what you saw” acts on the lifestyles ensuring blessings for the blessed life and curses for the unfortunated life.