James 2:13 (KJV) For he shall have judgment without mercy, that hath shewed no mercy; and mercy rejoiceth against judgment.
In other words, we will not receive mercy but be judged by GOD if we do no show mercy to others but if we show mercy to others we will be shown mercy.
This reminds me of two things Jesus said:

1. When His disciples asked Him to teach them how to pray He taught them the LORD’s prayer. One of the parts of the prayer is forgiveness: “Matthew 6:12 (KJV) And forgive us our debts, as we forgive our debtors.”. This He followed up with an explanation after the prayer indicating how important it is: “Matthew 6:14-15 (KJV) For if ye forgive men their trespasses, your heavenly Father will also forgive you: 15 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.”
2. When Jesus taught the parable of the unforgiving servant. Have a read of Matthew 18:21-35. In a short summary one servant had a big debt and asked for mercy and the king forgave the debt. Then the servant pursued another servant for a very small debt and did not have mercy. Therefore the king reneged and did the same the servant did to his fellow. In conclusion Jesus said “Matthew 18:35 (KJV) So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.”

From this we see that we really should be merciful if we want GOD to be merciful to us. Because non of us is yet perfect and we don’t have the right to throw the first stone (which Jesus Christ has) – see John 8:3-11.
It is easy to judge others and be lenient on ourselves because we know our circumstances and love ourselves. If we really love others we would show mercy and not quickly judge them and hold their mistake against them for the rest of our lives. In the same chapter of James it is written “James 2:8 (KJV) If ye fulfil the royal law according to the scripture, Thou shalt love thy neighbour as thyself, ye do well:” Therefore showing mercy is demonstrating love of our neighbour which is what our LORD wants.