The idea of testimony can be seen throughout the Bible. In Mark 5:19, Jesus told Legion to go home and tell others about how God took away his demons and restored his sanity. David proclaimed God's faithfulness, majesty, and righteousness throughout the Psalms, particularly Psalm 146.
Some people are shy about testifying the goodness of God in their life, probably they don't want to face crowd in the church or may be they think what happens to them is like a normal thing which must happen in their lives without praying or bringing in God.
But far beyond their imagination, testimonies goes a long way to solidify the miracle(s) in your life, it also help other believers to build their faith in Christ. God is also glorified when you share your testimonies and Satan the devil will be ashamed.
I remember when I had fire accident in July 11th, 2011 at about 2:30am, it so happened that my generator caught fire in my protected veranda, the only escape route was where the fire started and there was no way I could have escaped being burnt. Only God knows what would have happened. The only thing I did then was to open my window and scream for help and God being God, neighbors came out and quenched the fire after 30mins of their effort, then I was rescued from untimely death. I didn't keep it to myself but I called on friends, family and relatives to join me in thanking God and it has been a yearly routine.
In conclusion, always make thanksgiving an habit in your life as it goes a long way to open other doors for your greatness in life.