When Do We As Parents Become The Enemy???

In life we grow up get married and have children.We have children to love and mold into good people to be better than us.Sacrifice for them,get them the things they want and need,take care of them,cherish them like no other,clean up after them then somewhere along the way you become the enemy?? I don't understand why or how this happens? I know you are saying well they are kids one day they will realize and change, but until then how do you deal with the constant heartache,disrespect and reject? You think to yourself well once they are grown and move out things will change they will realize but no at 2:00 AM you receive phone calls,texts,e-mails etc to tell you what an awful parent you are and how you don't love them all the while they are cursing and disrespecting you. When did simple love become not enough and make you a bad parent? I simply don't understand and maybe I never will but until then I will continue to love and endure the pain of being a parent in the hopes that one day they will realize and understand that being a parent isn't always easy but we do LOVE you.