[Quick note: I would appreciate it if anyone could help me modify my blog template in order to make the text column wide enough to accommodate putting the comics in-line. The way it is now, it either clips off the comic, or displays it in such a way that the layout looks really ugly.]
I heard when I was younger, from... someone... maybe a sitcom... that it is a bad idea to owe a friend money. I did not understand exactly why, until I owed a friend money I couldn't pay back within any reasonable length of time. It ended up destroying our friendship (though I did, in time, pay back the money). An additional lesson here might be that you shouldn't borrow money you can't pay back, especially when it's from a friend.
But I'm going to talk here about loaning money, even if it doesn't specifically apply to Shortpants or my former friend. I am going to turn to the Bible for this. Though I am mostly an atheist, I think there is a lot of good stuff in the Bible, if one thinks of it in terms of "just plain good advice" rather than "do this because God said so." (And really, I think the "because God said so" parts are there for the obstinate douchebags who want to respond to everything with, "Yeah, well why should we?")
In the Torah, you are allowed to charge interest to gentiles, and profit from them, but you are forbidden to do this to another Israelite. This is mentioned in different ways in Exodus, Leviticus, and Deuteronomy.
In Luke 6:34-35, during the Sermon on the Mount, Jesus says, "If you lend to those from whom you hope to receive, what credit is that to you? Even sinners lend to sinners, to receive as much again. But love your enemies, do good, and lend, expecting nothing in return." (This passage does not appear in the other gospels, as far as I can tell.) On one interpretation, Jesus is coming along and saying, "When you lend, don't charge anyone interest, no matter who they are." That interpretation is substantiated by his general message that when you are to love your neighbor, that means everyone, even if they're a Samaritan or some other ethnic group people hate.
Taking the passage literally,* though, Jesus is saying much more: lend to anyone you can, and don't expect anything in return, not even what you loaned. There are Christians in the past and today who have taken the stronger reading, though I couldn't tell you who, because I have no idea where I would find that sort of information.
Suppose you think of it this way when you loan money: you say, "Sure, you can borrow $100. Just make sure you pay me back sometime." In your head, though, that is $100 you will never see again, so you should only do this if you can really afford to be out $100. Then, when your debtor really does pay you back, it's like they just gave you $100! You're like, "Wow, I'm going to buy so much booze with this money!" And isn't that awesome? It is totally awesome.
* - Beware of anyone who starts talking about a "literal" reading of the Bible. Most of the time this means taking things out of context, but I think you'll see that I mean something very different in my use of the word "literal."