The Scripture is filled with instruction to not fret:
Psalm 37:1 (ESV)
1 Fret not yourself because of evildoers;
be not envious of wrongdoers!
Psalm 37:7–8 (ESV)
7 Be still before the Lord and wait patiently for him;
fret not yourself over the one who prospers in his way,
over the man who carries out evil devices!
8 Refrain from anger, and forsake wrath!
Fret not yourself; it tends only to evil.
Jesus preached at length about the foolishness of worry:
Luke 12:22–31 (ESV)
22 And he said to his disciples, “Therefore I tell you, do not be anxious about your life, what you will eat, nor about your body, what you will put on. 23 For life is more than food, and the body more than clothing. 24 Consider the ravens: they neither sow nor reap, they have neither storehouse nor barn, and yet God feeds them. Of how much more value are you than the birds! 25 And which of you by being anxious can add a single hour to his span of life? 26 If then you are not able to do as small a thing as that, why are you anxious about the rest? 27 Consider the lilies, how they grow: they neither toil nor spin, yet I tell you, even Solomon in all his glory was not arrayed like one of these. 28 But if God so clothes the grass, which is alive in the field today, and tomorrow is thrown into the oven, how much more will he clothe you, O you of little faith! 29 And do not seek what you are to eat and what you are to drink, nor be worried. 30 For all the nations of the world seek after these things, and your Father knows that you need them. 31 Instead, seek his kingdom, and these things will be added to you.
The negative effects of stress and worry are well-known by most. Some recent research has found one more reason to not be anxious, cancer:
Monash University researchers have revealed that chronic stress builds lymphatic “highways” that provide cancer cells with a faster and more efficient way to spread.
The lymphatic system helps carry immune cells throughout the body in order to fight illness, however it also plays a role in transporting cancer cells throughout the body. A study by Dr Caroline Le and Dr Erica Sloan from the Monash Institute of Pharmaceutical Sciences indicates that chronic stress can be extremely detrimental to cancer patients. High stress levels increase lymphatic activity, making the patient’s own lymphatic system a more efficient and networked distributor of cancer cells.