He is not writing a love letter, even though he’s used the words “love” and “truth” more than once already.
Actually, scratch that – he supposes it could be a love letter, if you look at the words from a certain angle. Because he’s writing a letter of warning, it implies that he cares about the person he’s writing to, so having a little love attached to the letter is inevitable.
But he’d rather not be writing this letter, because letters can’t replace a person. Despite all the complexities of their language, words just aren’t enough to express the entirety of a human being, let alone what he or she wants to say. So every time he dips his pen into the inkwell, he imagines his soul as ink and writes, and hopes that the words he writes will come true so he will eventually be sent like a letter.