Why did they say no man is island?

yes,because every1 cannot live by our self

If a man were an island then he would have to create all the things that he needs to sustain or maintain himself without any external assistance or intervention. Man however is a creature of dependence and as a result cannot be (metaphorically) described as an island.

"No man is an Island" ...by that I take it your implying that man cannot exist without dependancy on others , or live alone without interacting with his fellow beings .

To an extent it is true , by our very nature we are "social animals" , however saying that , humans also ostracise themselves from the social group as punishment {for eg .. prison} then say if you get marooned on an Island , and have to live alone , without human company , such hardships are survivable , but ones quality of life would be somewhat diminished .

Overall , its nice to be in the company of good human beings , its just a saying , to remind people , not to get to introverted , not to leave society and at times those who care for you , when you get emotionaly upset . So like Yeah! I believe this saying has its merits .