because Jesus fasted. Lent is 40 days because Jesus spent 40 days in the desert being temped and fasting and praying before his public ministry began.
because disciplining our bodies disciplines our souls. By denying our bodies we are keeping ourselves from being controlled by our impulses. Our bodies and souls are connected and can’t be separated. One affects the other.
fasting keeps us from being enslaved to every urge in the body and lets our mind and soul be the master of our actions. Jesus showed us that having self-discipline allows us to follow God in total freedom.
Lenten Fasting Rules:
On Ash Wednesday and Good Friday, Catholics should eat only one full meal and two smaller meals which, combined, would not equal a single normal meal. Additionally, Catholics may not eat meat on these two days–or on any Friday during Lent.
These norms on fasting are obligatory from age 18 until age 59. The norms concerning abstinence from meat are binding upon members of the Catholic Church from age 14 onwards. Canon Law does mention that for young children not fasting, parents should still communicate the meaning and penance.
(NOTE: Children, adults with physical and mental illness, pregnant women and those nursing are all exempt. The USCCB stresses that “common sense should prevail” and that no one should jeopardize their health to fast.)
If possible, the fast on Good Friday is continued until the Easter Vigil (on Holy Saturday night) as the "paschal fast" to honor the suffering and death of the Lord Jesus and to prepare ourselves to share more fully and to celebrate more readily his Resurrection.
Do Sundays count for fasting?
It’s been a custom for Catholics to give something up during Lent, in addition to the fastings and abstinence rules. Some keep their sacrifice continuously, but Sundays during Lent are not “prescribed days of fasting and abstinence.”, as each Sunday is considered a “little Easter”. Ultimately, it’s a personal decision.
A Reflection on Lenten Fasting from USCCB (US Council of Catholic Bishops)