I can't select just one:

1. Chances of meeting even a single genuinely good person throughout the entire life are virtually 0.

2. There is no magic in relationship between different genders, there are no genuinely happy couples anywhere in the world (possible, but chances close to 0), everything about it in books and movies is fake (100%).

3. Humans are no different from other animals, they should never be sick, not even with a running nose, unless they are put in elaborate extremely hazardous environments/conditions to make them sick. Medics are not needed for anything other than fixing broken bones.