According to the author of 1 Kings, Adonijah talked to Joab and Abiathar, who were important men, and they decided to help him become king. This made Adonijah happy because he had powerful friends on his side.
1 Kings 1:7: And he conferred with Joab the son of Zeruiah, and with Abiathar the priest: and they following Adonijah helped [him]. [he...: Heb. his words were with] [following...: Heb. helped after Adonijah]
This verse emphasizes trusting in the Lord with all one's heart and not leaning on one's own understanding, which contradicts the actions of seeking allegiance through political alliances as seen in 1 Kings 1:7.
Proverbs 3:5: Trust in the LORD with all thine heart; and lean not unto thine own understanding.