The longer Tom refused to confide in Harry the more irritated Harry got. Tom shouldn’t be doing anything shady in the first place, but if he was going to do it anyway he should tell Harry! If he was feeling charitable – which he wasn’t – he could see that Tom would hardly be incentivized to take Harry into his confidence if he was planning something he knew Harry would disapprove of, because then Harry would be morally obligated to try to stop Tom, and Tom wouldn’t want to deal with the bother. Which was logical.