The titles appear bold until you read the thread. When someone adds new posts to the thread, it will appear bold again until you've read the thread again, then it will appear lighter again. Does that make sense, LOL?
So the titles to your own threads will appear light (since you read your own post when you were writing it) until someone adds a post, then it should appear bold.
Older threads will also be unbolded, whether you've read them or not.
It's a feature to help you see which threads you haven't read, or which threads have had new posts added since you read it.
Hope that helps.