Lack of self-control brings misery
"Like a city whose walls are broken down is a man who lacks self-control" (Proverbs 25:28).
One who gives full vent to his anger acts like a fool; one who keeps himself under control is wise
"A fool gives full vent to his anger, but a wise man keeps himself under control" (Proverbs 29:11).
Every Christian can be self-controlled; it's a fruit of the Spirit
"But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law" (Galatians 5:22-23).
We are commanded to be self-controlled
"Therefore, prepare your minds for action; be self-controlled; set your hope fully on the grace to be given you when Jesus Christ is revealed" (1 Peter 1:13).
One can and must develop self-control--put a lot of effort into it
"For this very reason, make every effort to add to your faith goodness; and to goodness, knowledge; 6 and to knowledge, self-control; and to self-control, perseverance; and to perseverance, godliness..." (2 Peter 1:5-6).