For 2 simple reasons:

1) Rh incompatibility

This is when a woman is rh negative, carries the antibodies and carries an rh positive fetus.

http://rhesusnegative.net/rhdisease/

How do you avoid it? Simple. If you are an rh negative woman, look through our list of rh negative men. See someone you like? Great! Give him a chance. If you marry an rh negative man, your children can only be rh negative as well meaning you will not have any incompatibility issues during your pregnancy.

And rh negative men: There is a shortage and demand for you. GIve an rh negative woman a chance as well. She will appreciate this natural solution rather than having to deal with potentially major complications.

2) Blood type personality traits

Popular in Japan and Korea. And very real as well. Since there are so few rh negatives in either country, I have taken the liberty to highlight the personality traits of all rh negative ABO groups:

http://www.rhesusnegative.net/staynegative/rh-negative-blood-type-personalities/

For all the others, there is Wikipedia:

Blood type personality theory

Any questions? Ask below!