Each person has got in them parts which are stronger and some which are weaker (no so emphasized).
Some has got it stronger in the sexual part, others in the social part, others in the functional practical part, others in the emotional part, others in the soul, others in the mentality, others in their spiritual inclination.
Of course any one can have few qualities in different degrees of strength; some are very powerful, others a bit less powerful and others could be in varying degrees of weakness.
Now, compatibility is to be in love with someone who shares with you as many identical and leading qualities as possible. The most rare and difficult to find in compatibility are the spiritual, soul and mental qualities, simply because the higher is the quality the rarer it is (gold for example), the lower – the more common. So compatibility between two people who share powerfull spirituality, soul and mentality – would be difficult to find.