It's not in the star sign, it's in the astrology. Venus rules your love life and can indicate how you feel and act in relationships. It sheds light on what you tend to do wrong in dating, and what you need to make the prime connection. Your sun signs might not be compatible, but your Venus' could be and this will speak volumes to your connection and intimacy.
It's important to look at the big picture and not just one fine detail because everyone has a birth chart composed of elements different signs placed at different planets. All of the planets represent different things and reveal various attributes and insights about a given person.
Capricorn - Pisces Compatibility
Each person is made up of the four elements, but some can entirely miss certain elements -- every chart is different. An energy deficiency also reveals elements of identity. You need the element of earth to stay grounded and form real commitments. It's possible to have your sun sign in air, and have the rest of your chart in fire.