What is the tone of the poem “On Another’s Sorrow”?

"On Another's Sorrow" is actually a poem with a tone shift.
The speaker begins the poem with a tone that is, well, sorrowful (borrowing the importance of the title). He notes that when we care about people, we feel their pain. When we see another in grief, we seek ways to alleviate that pain. When a father witnesses his child weeping, the father is filled with great sadness. When a mother hears her infant groan, her spirit groans, too.
The tone begins to shift in the fourth stanza to one of hope. Just as we feel the sorrows of those we love and seek to help them through the pain, "He"

[sits] both night and day,Wiping all our tears away.

The "He," then is God, who became a man (referenced later in the poem) through Christ who also felt these sorrows. And because of this, we can have hope that grief is temporary and that God sits beside us as a great Comforter through life's trials.