My experience is that whenever I start thinking about my physical body I start feeling that I have to struggle to stay in the dream, and usually I lose grasp of the dream and wake up.

Also, I tend to be more on "autopilot" in lucid dreams;
I know that I am dreaming, but I tend to be more impulsive and be like "yeah, I am in a dream, time to rub my hands until the dream stabilizes --- okay, finished! now I will go here and try Dream Goal X!".