Relationships..... specially long lasting ones..... should be based on trust mostly. There are however, relationship which is based on common interest, attraction and many more, but this kind of relationship usually wont last. Humans need companionship..... people to joke around with when they are feeling down(humor)..... someone to hold their arms when they feel hopeless(romance)..... someone to stay by their side so that they will feel safe(dependability). Thus..... only when the 3 qualities are met..... a true 'chemistry' shall emerge between the two and that's what we call 'love'.