Of course climbers think about and prepare for falling and other dangerous scenarios. In the "heat of the moment," though, too much mind paid to the negative can certainly outweigh concentration on the positive.
In the same way that nauseating conversation can disrupt a meal, I've had a few of my attempts at technical bouldering problems disrupted by friends conversing nearby about climbing injuries. In one case, hearing talk about dislocated shoulders and elbow injuries while I was working on a problem involving odd shoulder and elbow orientation made progress very difficult.