Man chose to sin against God rather than serve Him. The Bible says...

"For all have sinned and come short of the glory of God." Romans 3:23

What does it mean to sin

against God? Sin is breaking God's law: "Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law." 1 John 3:4

We sin by doing what God has commanded us not to do or by not doing what God has told us to do.