There are different kinds of social isolation:
i) Two people may live together, but be emotionally estranged. That generates a paradox: physical proximity with psychological distance.
A textbook example is a dying marriage. The couple may have been madly in love at the time they tied the knot. But now the relationship is apathetic at best or antipathetic at worst. Aloof or disdainful.
ii) Then there's physical separation. Separation in space. Grown children who live out of state. An elderly parent in a nursing home. A father or son serving overseas in the military.
Often, though, there's a hopeful element to physical separation. In principle, the loved one is just a car ride or plane ride away. They are still available. Accessible. Either you can visit them or they can visit you.
iii) Finally, there's separate in time. A loved one dies. That's more oppressive than separation in space. In case of physical separation, that's often something under our control. It's within our power to see them if we need to or really want to. Hop in the car. Hop on a plane.
But separation in time leaves the survivor, the bereaved, powerless to do anything about it. Their loved one is inaccessible. They may despair of seeing their lost loved one any time soon. The loneliness is harder to take, because there's no end in sight. There's nothing they can do to hasten reunion. Every morning they must start all over again. The interminable routine. Alienated from their surroundings. Its very familiarity is painful. A constant reminder of what they lost.