The experience of fear in dreams is something that relates to being unable to face a deeper part of one's self. Typically, the source of the fear links to something within us we don't like.

That being said, there are two ways one could relate this to having not felt fear in your dreams in so long. Either you are comfortable with the flaws you find within yourself - a positive thing that allows you to face them and change them head on...or you simply lack the emotional response all together - a less than ideal trait that skews the view of negative flaws into apathy and prevents positive change from occurring. The catch is that whichever it is, it can be different from dream to dream. I imagine that's the case here, as you claim to have had a handful of nightmares in the past, so your capability to feeling the appropriate emotional response is not dead.