Fear causes loss of control
When I dream, it's about 7 times out of 10 a nightmare. I'm used to it, had them all of my life. I've come to value my nightmares, I wouldn't want them to go away as they provide valuable insights and inspirations. In fact, most of my nightmares while horrific, are not as scary as they used to be.
However they do get in the way of dream control. I'm still in the baby stage of dream control, it's something I've only just consciously started trying to do only a few months ago. It took me a long, long time to feel comfortable with my nightmares and I felt I had to start with that before moving on to trying to control my dreams (even though I had dabbled in it before).
I am most likely to see horrific, scary things when I am not lucid but sometimes when I am lucid my fears become intensified, and anything I do see is suddenly 10x scarier than it normally would have been. This is something else I have slowly been trying to become accustomed to. Thing is, every time I do experience the fear it becomes a distraction that causes me to lose control of the events in the dream and breaks my lucidity.
For example, if I see someone I love in a dream I will try to keep control over the dream to avoid them becoming distorted or messed up, sometimes by behaving around them as I would if I were awake and pretending for that moment I am awake with them. I usually prefer not to see anyone I love in a dream though, and the fear of seeing them messed up often causes it to happen. At that point, all control is lost as I just abort the dream or go into fight or flight mode and lose my dream awareness. It's like an instinct, and I often wake up wishing I had persisted and not given in to that instinct. After all, none of it is real so there really is nothing to fear.