Being proud is one of those bad things that are not accompanied with a "Thou Shall Not" commandment.
If you are wondering why you should not be proud, here are two good reasons from the Bible:
1. God resists the proud but He gives grace to the humble. (James 4:6)
2. Pride leads to destruction and a haughty spirit to downfall. (Proverbs 16:18)
We can see a good example of this from Amaziah who killed 10,000 Edomites in the Valley of Salt and captured Sela by war. He then sent a message to King Jehoash of Israel challenging him to a battle.
Jehoash replied by saying, "A thistle sent a message to a cedar asking for her daughter to marry his son and then a wild beast from Lebanon trampled the thistle."
King Jehoash continued and said, "You have indeed defeated Edom and have become proud. Glory in that fame but stay at home because why should you stir up with trouble so that you fall, you and Judah. (2 Kings 14:1-10)
But Amaziah would not listen and so he went to fight Jehoash. He lost and part of the walls of Jerusalem were broken down. In addition, all the gold and silver in the house of the Lord and the treasuries in his house were taken by the victors. (2 Kings 14:12-14)