Loneliness, on one hand, is a subjective feeling. It arises out of an unfulfilled need to be vulnerable with at least someone, to have that person listen to you with compassion and without judgement. And so, lonely people need compassionate listeners.
Social isolation, on the other hand, is a choice to disconnect from other people to avoid distractions and be by oneself. Socially isolating people need to be left alone, in peace.
Loneliness and social isolation may seem similar, but while the former is always out of necessity, the latter is a choice.