"Well..." That was a very good question, one that Sirius definitely deserved an answer to, but he couldn't tell him that he'd read what was clearly meant to be a private journal entry. "Just... there's been a lot going on lately, and with you and James still working through things... I just thought you might need a friendly ear."
Really, he should have offered before, even without the journal entry, but with everything else going on, he just hadn't thought to yet.