1. Maybe they gave up in dating after their previous relationships ended dramatically and they are so heart broken.

2. They are nice people but they are just happy being single.

3. Maybe they have serious illnesses since young and they do not want to burden others or possibly pass down the illness to their future offspring.

4. They just cannot find THE ONE. So rather than marrying a wrong person, and ended up divorced and live a depressing life, better stay single and enjoy life.