Why do you keep running into your ex and dreaming about him and missing him after 2 years?

Dreaming about him and missing him happen because you're obsessing about something that, realistically, is never going to happen. Give it up and move on.

Running into him happens because, well, if you live near each other and had similar enough interests and hobbies to date in the first place, odds are pretty good you'll both continue to go to the same sorts of places. It doesn't mean anything. In particular, it doesn't mean you're "meant" to be together. See the end of the last paragraph.