What does the Bible say about loneliness?

Being alone and being lonely are two different things. One can be
alone without being lonely, and one can be lonely in a crowded room.
Loneliness is, therefore, a state of mind, an emotion brought on by
feelings of separation from other human beings. The sense of isolation
is very deeply felt by those who are lonely. The Hebrew word
translated "desolate" or "lonely" in the Old Testament means "one
alone, only; one who is solitary, forsaken, wretched." There is no
deeper sadness that ever comes over the mind than the idea that we are
alone in the world, that we do not have a friend, that no one cares
for us, that no one is concerned about anything that might happen to
us, that no one would care if we were to die or shed a tear over our
grave.

No one felt loneliness more keenly than David. In a series of earnest,
heartfelt appeals to God, David cried out in his loneliness and
despair. His own son was risen up against him, the men of Israel went
after him, and he was forced to flee from the city, and leave his
house and family. Lonely and afflicted (Psalm 25:16), his only
recourse was to turn to God and plead for mercy and God's intervention
(Psalm 25:21) because his only hope was in God. It is interesting to
note that the word "lonely" is never used in the New Testament to
describe people. In the New Testament, the word "lonely" only occurs
twice and both times refers to desolate places (Mark 1:45; Luke 5:16),
where Jesus moved off into the wilderness to be alone.

Whatever the cause of loneliness, for the Christian the cure is always
the same—the comforting fellowship of Christ. That loving relationship
with our Master has reassured and encouraged countless thousands who
languished in prisons and even went to their deaths for His sake. He
is the friend who "sticks closer than a brother" (Proverbs 18:24), who
lays down His life for His friends (John 15:13-15), and who has
promised never to leave us or forsake us but to be with us until the
end of the age (Matthew 28:20). We can take comfort in the words of
the old hymn that says it best: "Friends may fail me, foes assail me,
He is with me to the end. Hallelujah, what a Savior!"