basically its saying ,you exist in anothers mind thats as real as it will get for the man in his mind,if he believes otherwise he is lying to himself,i think it was spinosa who said i think i am there for i am.

sounds like something Sartre would write, dolefully existentialist.

The quote is saying that when men tell themselves that no man is an island, they are deluding themselves.

everyone's an island. anything you feel for someone is all in your mind, anything you think they feel for you is an illusion that you let settle over you. at the end of the day, everyone's alone (and no one has a real friend, as pink floyd might observe)

it means most of the relationships we make are fairly tenuous.
We call people 'friend' but much of the time that word is a platitude rather than anything meaningful.

HunneyZ he DatinGz OtheRZ GuRLz
TimEZ u GoTZ Over ITz

It's true that our love stays strong when the memories the relationship are vivid.

I don't know the context of your quote but it maybe referring to existential nihilism.

Amongst the largest of crowds there is only ourselves

It means we live in a holographic universe

:)