It sounds to me like the couples (possibly long-term) relationship was teetering on a breakup. When the breakup happened and the girl asked him for a reason, he couldn't give her one because he really had none since he clearly didn't know what he wanted. Perhaps the guy was looking to see if he could find better than what he had (or he had a wandering eye for another girl) and he left her, only to realize too late that he had what he wanted all along. They may have gotten back together because she's willing to forgive him for what he did. The guy is flustered and afraid he will repeat the mistakes. He realizes he's completely ruined what they had and doesn't think things will get fixed(or rather, pieced back together). Even though she's willing to work with him, the guy doesn't even try to fix their relationship (out of fear) and leaves again. He then leaves a poorly written note for her. He knows well that she will see through whatever shoddy, dishonest reasons he gives her for leaving, but will not tell her the truth. He still leaves, fearing his mistakes, which in the long run will certainly come back to haunt him in the future. Thus, he is screaming out (figuratively and literally) his true feelings of love for this girl.