Friendliness is being open toward other people, taking the risk of inviting them into relationship with you. It means being curious, warm and inviting toward people you don't know well and letting yourself be vulnerable and interdependent with people you do.
When we are friendly, our starting assumption is that others are well-intentioned and open to reciprocity, and that we can learn from them. This does not mean we are naively oblivious to the fact that generous assumptions may be wrong--and definitely will be at times. Rather, friendliness means that in the absence of evidence to the contrary we assume the best, and even when evidence is mixed we tend to give people the benefit of the doubt. Because our expectations are often self-fulfilling, friendliness maximizes the richness of our relationships.