Everyone is alone in the world, separate from all else, at all times, and never truly capable of saying with certainty that things could be otherwise. This is both a fundamental existential problem and a flawed way of looking at human relationships. It is true: each individual is separated from the world by his or her perceptions, but: there is a reason why human beings cooperate, why we integrate ourselves into larger social fabrics, why we maintain relationships from birth to death, or for as long as possible.
We are “social beings” is a common way of saying it. The human being is the “grammatical ape”, a talkative species that uses codified sounds to create and transmit meaning and to build a community of individuals, ideas and voices, in which the individual can benefit from having connections as well. The “human” is an idea, not a fact, another way of looking at things, and so we should not even go as far as to say that the individual is apart from everything else, as we cannot define totally what it is that makes us a group in which that is true, aside from DNA and appearances.