Dani has a past better left hidden. How is she supposed to maintain that with the Cowboy Bounty Hunter running loose on the Texas Ranch in which she manages. Having Jack around is a good thing when it comes to protecting her guest, but a bad thing when it comes to her past. Jack sees things in simple black and white. He sees no shades of gray.
Jack is hiding things as well so what could that possibly be? Who is he?
Jack trust no one until the prove to be innocent. He will find his criminal at all cost and any others that get in his way. He can't get involved with Dani because he has nothing to offer her. His quest for his brother's killer takes up his entire life. Once he catches them he MAY be able to move forward with his life and allow other people into his world.
Everyone makes mistakes judging someone when your own path isn't so clean is hypocritical.