Some spoilers ahead!
There are no Strings on me so. . . . . You were Warned.
Previously on the 3rd installment of the Iron Man Franchise, Tony Stark decides to be a "Changed Man" after defeating Aldrich Killian A.K.A "The Mandarin" and seems like he want to establish new life with Pepper Potts, that's why at the end of the film he decided to remove his Arc Reactor chest piece and start to live like a normal human being again (Kinda?). So some of the fans wondering after watching IM3, What's the sense of being Iron Man without the iconic Arc Reactor?
After Thor: The Dark World & Captain America: The Winter Soldier and to continue MCU Phase 2 by the Most anticipated movie of the year Avengers: Age of Ultron has come for answers but as well as the previous films, the fans didn't get any answers and still wondering about it. Even me.
In the Post Credit scene of IM3, Remember When Tony is having a conversation with Bruce Banner talking about past? by the way! TAKE NOTE, this is just a clip. Maybe after this scene both of them are already having their brainstorming ideas about Artificial Intelligence (A.I) project that they were about to developing and later on they called The Ultron Program, it is possible that if the program works out as planned, Tony can retire as Iron Man and leave the Avengers to Captain America's honorable command but unfortunately they created their own enemy and turns out to be a Menacing, Intelligent, Self-developing Evil Robot villain that AGAIN threats the safety of all mankind.
And of course! The Avengers will not be The Avengers without this genius.