Their fear hinders them from pleasing God because they waste their efforts appeasing other people. He knew of Paul's frequent trials and persecutions. He knew that he, too, might suffer those same persecutions. Paul reminded his young colleague that fear of others does not come from God. Fear causes us to stop and question what God has clearly told us to do.
Most fear is fear of the unknown.
We do not know what lies ahead of us, so we become apprehensive. Our imaginations can magnify problems until they seem insurmountable. We need a sound mind to see things in proper perspective. That is why God gave us His Holy Spirit, to enable us to see things as God sees them. Fear is no excuse to disobey God.