David was thirty years old when he became king, and he reigned forty years

2 Samuel 5:1-16

All the tribes of Israel came to David at Hebron and said, “We are your own flesh and blood. In the past, while Saul was king over us, you were the one who led Israel on their military campaigns. And the Lord said to you, ‘You will shepherd my people Israel, and you will become their ruler.’” When all the elders of Israel had come to King David at Hebron, the king made a covenant with them at Hebron before the Lord, and they anointed David king over Israel.

David was thirty years old when he became king, and he reigned forty years. In Hebron he reigned over Judah seven years and six months, and in Jerusalem he reigned over all Israel and Judah thirty-three years.

The king and his men marched to Jerusalem to attack the Jebusites, who lived there. The Jebusites said to David, “You will not get in here; even the blind and the lame can ward you off.” They thought, “David cannot get in here.” Nevertheless, David captured the fortress of Zion—which is the City of David.

David then took up residence in the fortress and called it the City of David. He built up the area around it, from the terraces inward. And he became more and more powerful, because the Lord God Almighty was with him.

After he left Hebron, David took more concubines and wives in Jerusalem, and more sons and daughters were born to him. These are the names of the children born to him there: Shammua, Shobab, Nathan, Solomon, Ibhar, Elishua, Nepheg, Japhia, Elishama, Eliada and Eliphelet.

Why?

David became the king of all Israel, conquered Jerusalem, and later became the nation's capital. Because the Lord Almighty was with him.

David took more concubines and wives in Jerusalem, this is the reason a few years later there were wars among his son. It has begun desolated in his life at Jerusalem.

When we fear the Lord and obey the word of the Lord comes with peace and strength, if has an easy life, and sexual behavior, then war and troubles come. 

Solomon was born in Jerusalem.

How?

Trouble and difficulty make us more trust in the Lord, deluxe and rich are easy turn to destroy and desolation.