The dictionary says bitterness is the state or quality of being bitter; figuratively, extreme enmity; sharpness; severity of temper; biting sarcasm; painful affliction; deep distress of mind; animosity.
Hebrews 12:15; "Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled". The root of bitterness refers to a spirit and thoughts that is described by intense animosity and resentment.
Bitterness can be directed towards anyone such as someone in the church, workplace, school, home, or even in your neighborhood. It can also be a bitter resentment toward God because of His discipline. One needs to remember when God disciplines us it is not because He doesn't love us, it is because He does love us.
When a person becomes bitter it results in defiling that person. The person needs to go immediately to the Lord in prayer and ask Him for the strength to overcome this bitterness. If a person doesn't work on this bitterness it can continue to grow and grow. Before you know it you have hate in your heart for this person or persons. Then the bitterness and hate will cause you to have an unforgiving heart. Bitterness can even reach out and touch other people. This bitterness can create bitter fruits to others. Then before you know it the bitterness has caused you to have insincere principles. The more bitterness, hate, and an unforgiving heart you have the less time you want to spend with the Lord. Now, you have lost the fellowship with the Lord and you have walked away from the Lord. Where are you going to spend eternity now?
Whatever has started a person feeling bitter, it needs to be taken care of immediately. Bitterness leads to so many things and can cause a person to walk away from the Lord. There is nothing so bad that we should allow ourselves to carry bitterness around.