Acts 13:22
And when he had removed him, he raised up unto them David to be their king; to whom also he gave testimony, and said, I have found David the son of Jesse, a man after mine own heart, which shall fulfill all my will.

Why was David a man after God's own heart? This was because David found favour before God. God favoured David above so many, many people; too numerous to be counted.

Acts 7:45-46
...unto the days of David; Who found favour before God, and desired to find a tabernacle for the God of Jacob.

Why did God favour David so much to the point of calling him "a man after mine own heart?"

One reason was because David was a worshipper extraordinaire; he was a man whose fulltime job was to praise God and David did this excellently throughout his days and as king of Israel. In short, God loved David so much because David was such a commited worshipper.

Psalms 119:164
Seven times a day do I praise thee because of thy righteous judgments.

Psalm 34:1
I will bless the LORD at all times: his praise shall continually be in my mouth.

Psalms 119:62
At midnight I will rise to give thanks unto thee because of thy righteous judgments.

David praised God throughout the day; in the morning, afternoon, evening and at midnight.
David was so devoted to worshipping God, he appointed fulltime musicians to minister to God throughout his reign as king:

1Chronicles 6:31-32
And these are they whom David set over the service of song in the house of the LORD, after that the ark had rest.
And they ministered before the dwelling place of the tabernacle of the congregation with singing, until Solomon had built the house of the LORD in Jerusalem: and then they waited on their office according to their order.

David also made musical (or invented?) instruments to praise God! David himself was a musician and loved praising God with different musical melodies.

2Chronicles 7:6
And the priests waited on their offices: the Levites also with instruments of musick of the LORD, which David the king had made to praise the LORD, because his mercy endureth for ever, when David praised by their ministry; and the priests sounded trumpets before them, and all Israel stood.

Because David so excelled in praising God far beyound his contemporaries, God favoured and blessed David greatly. God did the following mighty things for David and He will too for us if we will praise Him more often.

1. God showed David mercy.
Acts 13:34
And as concerning that he raised him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David.

If there was a man who enjoyed divine mercy in the Bible, it had to be David. David faced a lot of dangerous and difficult situations, but because God had mercy on him, he came out unscathed.
Mercy is very important because unless God mercifully intervenes in some difficult situations in our lives, we may die with them.

In 1 Chronicles 21 when God gave David 3 options of punishment for disobeying him by counting Israel in a census, David choose a destroying angel because he knew God would be merciful to him. The angel was to destroy the people for 3 days, but out of mercy, God changed His mind and ordered the angel to stop in the first day as the angel went about destroying.

2. God promoted David above King Saul.

2Samuel 12:7-8
And Nathan said to David, Thou art the man. Thus saith the LORD God of Israel, I anointed thee king over Israel, and I delivered thee out of the hand of Saul;
And I gave thee thy master's house, and thy master's wives into thy bosom, and gave thee the house of Israel and of Judah; and if that had been too little, I would moreover have given unto thee such and such things.

Out of favour God lifted David above king Saul and gave him his throne and his wives. There is really no limit to how much God can lift, because He is the most high. He sits above all rulers and leaders no matter how highly placed they are and can dispose them anytime He wants.

3. God enabled David fulfill his destiny.
Acts 13:22
And when he had removed him, he raised up unto them David to be their king; to whom also he gave testimony, and said, I have found David the son of Jesse, a man after mine own heart, which shall fulfil all my will.

What was David's destiny? It was to be a great king over Israel. God against all odds enabled David to fulfil his destiny even though he faced many death sentences.

4. God made an everlasting covenant with David.
2 Samuel 23:5
Although my house be not so with God; yet he hath made with me an everlasting covenant, ordered in all things, and sure: for this is all my salvation, and all my desire, although he make it not to grow.

What was the everlasting covenant God made with David? God covenanted with David that he will never lack a son that will sit on his throne forever. Jesus was born to fulfill this promise.

5. God cut off all his enemies and made David a great man.

2Samuel 7:9
And I was with thee whithersoever thou wentest, and have cut off all thine enemies out of thy sight, and have made thee a great name, like unto the name of the great men that are in the earth.

Are you a worshipper? David was, and his life is a testimony to the fact that God indeed is seriously looking for worshippers!