Psychological Comfort Zone
There is always a common thread capable of weaving into a bond of understanding between two people of the same sign and although sometimes subtle, it will be there for the parties to build upon or to deny.
There are also the natural affinities and antipathies between people of different signs because of diametrically opposed points of view, of themselves and of the universe around them. Awareness of the fundamental differences allows choices for contact with or avoidance of some personality types.