...what motivates a mentor to help a beginner anyway?
I view mentorship is a long-term commitment - one that I'm not currently making anywhere. But if you mean, "Why does someone occasionally answer questions on the forums?", then to analyze myself, it seems to be a mix of:
A) I'm at a boring part of my code, or I'm exhausted from doing some physical labor and don't want to code, or I'm waiting for a question *I* asked to get some responses, so it's a way to entertain myself while still tricking myself into thinking I'm being productive.
B) Someone else was answering the beginner's question... and they were wrong.
C) Pride. It's a bit of, "See, I know something!"
D) As much as I hate to admit it, the reputation system as an extrinsic motivator does have a small but noticeable effect. That's why I'm not as active on Stack Overflow - I find myself getting too caught up in the, "I got a point!" of SO, so I force myself to not go there too frequently. With GameDev.net, it's more like what you mentioned, validation that my info or contribution was correct. We actually were discussing this on the game design subforum a week or two ago.
[A] gets me to the site, [B] gets me posting (because of [C]) if [A] didn't already, and [D] creates positive reinforcement.
I also view writing an article as a huge commitment of time - because I feel it articles must have a high level of quality.
My previous post above this one took me an hour to write, because I think it through alot. Writing this article took me an entire day.
I have some ideas for a few more articles to write, but man, the time investment for the "quality" I demand of myself wards me off from doing so. But then I waste just as much time lurking and posting in the forums anyway.