I make a very logical rule to lend money to someone but they have to pay that back before I lend more. It's not at all unreasonable and I don't feel bad stating it. To heck with the "thank you"; I would cite that as another reason not to lend him more; if he has to be reminded to be polite and grateful and is taking you for granted? I don't think he'd get more of my money and I'd start bugging him so hard for repayment he'd be avoiding me :-)