Ok he wanted to escape the troubes of life with his wife, thats why he tried inception. Well, I think the entire movie is Cobb stuck in his own reality loop. The totem Cobb was using was that of his Wife, he did not have one of his own, that was purely her idea and he even said so himself. As he said, never let anyone else feel the weight etc if your totem, it's what locks you to reality. With his not having a totem of his own, the totem actually becomes useless completely. As a result, instead of it signifying whether or not he is in a dream like he thinks, it signifies what he wants to believe. So it topples over when he wants to believe he's in reality, and it keeps spinning when he wants to believe he's dreaming.
He believes he accessed his Wife's locked up totem so he can implant an idea for her... but I think she allowed him to access her totem so he could think he was the one implanting the idea. He mentioned that the only way for inception to work, is if the person thinks the idea is truly their own. He thinks he was the one who incepted the idea to his wife that they were trapped in a dream state, when it was really her doing the inception. The problem is, he had no lock on reality like she did. As she kept trying to draw them further out, he was consumed by the idea that they had come out of the dream, thus he didn't join her when she jumped off the ledge to get to reality. He then gets trapped in his own loop. This was also signified by the scene with the Mirrors, where he is looking into the infinite reflection of himself.
In the beginning, Saito sees the totem Cobb was carrying and he remembers back a long time ago, he once knew someone who carried an object like that. At the end, Cobb uses the totem to try to point out to Saito they are in limbo. In that first scene, Saito was actually referring to that end scene. Thus showing the closed loop Cobb himself is trapped in (Nolan has used this trick in a couple movies, when he filmed Memento and The Prestige). You can tell they are still in a dream at the end, as Cobb's kids are still wearing the exact same thing, and even doing the exact same thing he last remembered. That in and of itself made him suspicious enough to spin the top once more... but his having finally been able to change his memory of them (as he had tried to do several times through the movie) allowing him to see their face... he no longer cared if it was a dream or not. That's why he walked away from the top without checking to see if it would topple over (which would ultimately mean nothing anyway since it did whatever he wanted to believe it would do since it wasn't his totem
Just watched the movie like 3 hours ago