I'm thinking about the time I was standing on the street corner and someone ran right past me.
What made you think about that just now?
I guess it occurred to me that my ex-boyfriend lived on that same street corner.
It's normal for things to remind us of ex lovers.  Just make sure that you are able to move on with your life, and not get caught up on thinking about them too often.
Why? I have nothing better to do.
It's fine to think about your ex, and it's normal for that sort of thing to pop into your head when your mind isn't otherwise occupied. I'm just saying that you shouldn't dwell on past relationships too much, as the thoughts may detract from your present life.