Aggressive men

Why is it that when men get comfortable in there relationship and has been with a women for over a year or more they feel as If they own her and that there is no need for them to do the things they once did to keep her they feel as if they can tell her what she can and can't do or who she can and can't hang with ...get really aggressive and controlling ?? And when you threaten to leave them they swear up and down they'll change ..