Why does the green bar on my armour decrease over time?

Taken from Minecraft Wiki

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Armor like tools and weapons contain a durability. It represents the number of useful actions an item can perform and depletes upon item use. For tools and weapons, item durability represents the number of available uses before the tool is destroyed. For armor, it represents the amount of damage that can be absorbed before the armor is destroyed.

The remaining durability of any item can be seen by looking at the item's durability bar on the bottom of the item icon in the inventory and action bar. An item that has not once been used will not display a durability bar. As the item's durability decreases, the bar shortens right to left, changing color from green to red. When the item has only small number of uses left the durability bar is an empty gray line.

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Armor durability

Armor durability is based on the armor's type (head, torso, legs, feet) and material (leather, gold, iron, diamond). Any time you take damage, each piece of armor you are wearing loses one point of durability. Armor durability only decreases when its wearer takes damage that the armor is capable of reducing. This includes: Direct attacks from mobs and other players Getting hit with an arrow, snowball or fireball (either Ghast or Blaze) Touching a block of fire, lava, or cactus Explosions The following types of damage are not reduced by ordinary armor and have no effect on the armor's durability. Some enchantments can protect against them, but they still don't damage the armor. Ongoing damage from being on fire Suffocating inside a block Drowning in water Starvation Fall damage Falling into the Void Poisoning e.g. from a Cave Spider bite or a Potion of Poison Instant damage from a Potion of Harming Values represent the number of points of durability damage this armor must take before it is destroyed. Note that every time the player takes damage that armor is capable of reducing (see above), it counts as one point of durability damage for every worn armor piece. Armor with the Thorns enchantment automatically loses another point of durability, and two more (for a total of four) if it reflects damage to the attacker.