Actually, differences are important in a relationship. If there are few of them, both will be bored with one another. Each Zodiac sign has one of those qualities. Capricorn in cardinal, which means leading, even rather bossy. They are Aries, Cancer, and Libra. None of them is that compatible with Capricorn, since there will be a rivalry of leadership. Two strong wills often collide.
Capricorn is much more compatible with the mutable signs, which are the followers of the Zodiac. They are Gemini, Virgo, Sagittarius, and Pisces. Such a relationship, with one leader and one follower, is at least mostly smooth. But of course, there can be other issues. If not, the two may sooner or later find that they are incompatible.
The fixed signs are Taurus, Leo, Scorpio, and Aquarius. But nothing is written in stone. A relationship is complex. There is no guarantee as to what will work and what will not, especially at length.
All the personality traits of each Zodiac sign have to be considered when examining their compatibility in astrology. Generally speaking, Capricorn wants a relationship to lead to something — the bigger the better. Family, home, a bunch of children — everything that can be described as success in life.