No one has ever asked me to borrow serious money so my experience is limited.
1. Never lend money
If you think of it as giving money to someone, regardless of whether it is their intension to pay you back, you won't be disappointed.
2. I frequently lent money to my mother when I was in high school.
At some point I became fond of saying "you're never richer than when you're in high school, have a job and live at home." I worked at that age and I always had a few hundred dollars. Not a lot but for getting around, buying beer, gas and dates, I was in good shape. We had real home problems and I pitched in regularly, I think the money I lent my mom covered groceries but I am not certain.
3. Be part of their solution
I don't know what I would do if someone did ask for serious money but part of my response would want to be to help them diagnose the issue and help them figure out a solution which I am sure would be a turn off to quite a few who would be inclined to ask.
4. Crazy pet peeve
Because I think it is related. Back in 2008, I was best man in my best friend's wedding. We were all early 40's. I planned a bachelor party where we went to a SF Giants game in the afternoon and then a Warriors game at night. I bought all the tickets and two of the guys after acknowledging up front that they would pay me that weekend ended up stiffing me. The Pet peeve is not doing what you say you're going to do.