The day when this letter is written is one lovers hope would never come.
A goodbye letter from a lover does not come only when one partner ditches another, but can also be a result of an unavoidable separation or mishap. A letter to a soldier going to war gives a fair idea of what we are trying to imply.
Anyhow, goodbye love letters are testament to the fact that even though the paths are getting separate, there is still that undying light of love that binds the two lovers. It is a symbolic gesture of all the feelings that make love such a fantasy. You do not need to be formal in these sorts of letters; after all they are nothing but the expression of the heart and reflection of your soul. Salutations and other formalities are not required.