We are born in this world through a natural process and become the children of parents as related to us .In a similar process, our siblings are borne and become related to parents.We become related to our siblings by virtue of being children of same parents or more specifically of same mother and become blood relations...The hierarchy of blood relations get extended through genetic or blood relationships producing legal heirs or coparceners in the universe... linking the ancestors with the descendants...
Brother,sister,mother,father,cousin,uncles, aunts ,etc become blood relations.
One gets married to another person. Here the relationship prevailing is one caused as a result of legal contract of marriage between a man and a woman.
We come across many persons in the world. We do not closely observe and feel the divine qualities or characters of our siblings or Spouse or parents, or our descendants. We are blessed in this world for being in the midst of certain divine persons who are not our blood relations..We realise and recognize the said divinity...in male or female,on account of his or her nature , character, appearance, activities, level of knowledge, attitudes,tolerance,patience ,faith in God, etc. as perceived by us .We preserve and cherish them in our minds and hearts as God and feel personally associated with them. We wish to remember them,like to see their prosperity and desire for their welfare . We feel hurt and unhappy when such individuals are subjected to any type agonies in life. We deem them as Divine and may be we might not have spoken to or physically seen such messengers of God. ..as we donot see God Almighty in person or speak directly to God. Of course we can see God in person after developing faith in Him ,praying Him always with devotion and realizing his greatness and submitting to Him for attaining Moksha...
Now a group of persons of various interests become our friends in Facebook..we see them through profiles, read their writings, see their family, events ,etc but would somehow feel them as unrelated social net work friends. Many of them would not have seen us in person and vice versa .We get option to block or unfriend any of them anytime .
May be there will be other definitions for friendships and relationships But it is absolutely certain that human beings need friends and relations of categories mentioned by me for existence...
Unknown we meet many but known got mingled and live to depart known to meet again... with a wish to be born as same siblings,parents,children, descendants, etc....