What’s Up With the Title?
Here's the deal. All 154 of Shakespeare's sonnets are assigned an individual number instead of a title. Sometimes they're referred to by their first lines so, #146 is often called "Poor soul, the center of my sinful earth."
We have no idea who came up with the order of the sonnets because Shakespeare probably didn't have anything to do with their publication. Ordinarily, we'd tell you how important the order of the sonnets is because the overall sequence unfolds like a juicy soap opera. (Think betrayal, love triangles, fights with BFFs, professional rivalry, steamy hook-ups, etc.)
But Sonnet 146 is different because it doesn't seem to involve any of the other recurring characters, so to speak. (Except for our speaker, of course.) For instance, Sonnets 1-126 all seem to be addressed to a young man critics like to call the "Fair Youth." Sonnets 127-152 seem to be mostly about the Speaker's steamy affair with a two-timing mistress who treats him like garbage. Sonnet 146 appears to be talking to the speaker's own soul. Odd.