From "So you do really care" and his well-established view that most people are painfully stupid, he should deduce also the latter, as it is more unlikely that Harry is both exceptionally rational and exceptionally caring unless he has a reason to believe that the former causes or at least strongly correlates with the latter.

Then again, someone who has a low opinion of others' intelligence should already believe that others are not rational enough to seek resurrection, even if they cared to want it.