Why dragon is the most dangerous man in the world.

My theory is that while Dragon was in the Marines, he was a bodyguard for Imu, where they fell in love and had a child (Luffy). After finding out about the evils of the world government and Imu (possibly from Kuma) he ran away and started the revolutionary army, later returning to kidnap Luffy who he left with Garp. (Not abandoning him) to protect Luffy so that Imu would not know that Luffy was the stolen child, thus hers as well.

After Dragon stole the baby, Imu gave him the highest bounty in retaliation.

I also believe Imu probably figured it out, and that's why Luffy is still alive and why admirals like Kizaru are sent after Luffy, knowing that he would not take the fight seriously.

Knowing now that Luffy ate the Nika fruit, I highly suspect that Imu will make herself known to Luffy and either try and get him on her side, or use Luffy to awaken the weapons knowing that he may be the only person who can put an end to her reign.