Why did my ex do this..?

It sounds like one of 2 things.

Number 1: He wants you as his back up plan. In other words, it is nice for him to know if he can't find someone else that you will still be around. If this is the case you are better off with out him.

Number 2: He is just plain flaky or he has problems making commitments. If this is the case, if you really really care about him, you should suggest going to therapy to figure out what makes him acts this way, and how to control it. If not keep him out of your life.

You did not mention an important item, when he comes back do you have sex with him? If you do, that could be the answer...he wants some sex, and will say whatever to get it.

Get a man and not a game playing boy. Stop wasting your emotions on this crap

Men want a fun, self-confident gal who respects herself and who doesn't need him to have a good time.. and this guy sounds like a retarded person
Not only should you respect other people, but you should demand the same degree of respect for yourself. You should never put up with any kind disrespect from anyone, especially a person you are in a relationship with. You will have a much better chance of keeping a woman interested in you if you stand up yourself, than if you allow him to walk all over you.