Minor Spoilers with this question, What would you write in a good bye note when leaving your spouse?
I think the wife left the note blank, because what is there to really say? She left him and nothing she says for the reasons why are gonna make it better? At least I believe thats why she left a blank note.
I think she left him because she felt unfulfilled for some time and then their Son overdosing on smack (at least thats what she thought, she never discovered the truth) was the final depressing event that had her wanting out completely.
The film works the best with her out of the picture though.