Is it selfish to refuse to lend your Airbnb to friends?

No, it's not selfish at all! It's your investment, your property, and you have the right to do whatever you want with it! After that, I get that it can be awkward to turn down your mates, but you can always offer them a nice discount if you want to have them over, that changes things! In the end it's your money and your responsibility if something goes wrong, so you've gotta be comfortable with your decision.