Theory #6: Cobb was stuck in a closed-loop state designed by Ariadne.
This theory explains Theory #5 in more detail and should be read as complementary to the latter rather than in isolation. The involvement of Ariadne in Cobb’s inception mission should not be underestimated as merely utilitarian. She was more than just an architecture student of Miles or (as we later found out) a skilled builder of dreams. In fact, she was probably the only one besides Miles himself who knew about the “inception-within-an-inception” mission.
In (2) - the inception mission on Fischer - Ariadne’s role was to help Cobb design the levels of dreams. However, in (1) - the inception on Cobb - Ariadne’s role was to act as Cobb’s psychoanalyst. She was to help in providing emotional support, and to further understand his troubled psyche, especially about his problems with Mal, with the sole aim of providing him with the means for catharsis, and more importantly, to slowly gear (or manipulate, if you will) him towards voluntarily accepting the idealized “state of being” that he would eventually find himself in at the end of the film.
This idealized state could be described as a closed-loop state designed by Ariadne to contain Cobb’s mind in a place where he could be happy forever, like a dream that ran for eternity. Furthermore, with Cobb’s mind already being incepted with Miles’ idea of reunification in the film’s last sequence, it was likely that Cobb had accepted his perceived state of being as real and genuine as opposed to a sense of “false consciousness”.