Why is the experience in " A Great Pain" described as " a formal feeling"?

Because like many formal occasions, it lacks true feeling. Many times at formal occasions we just "go through the motions." The rest of the first stanza supports this with words like "ceremony" and "stiff." The connection is made even stronger with the addition of the the entire simile "ceremonies like tombs." just think about th mechanical nature that the bereaved move through at a funeral. This is again reinforced at the beginning of stanza 2 with the description of the feet as mechanical.