I would like your opinion on what might be going on with my friend, if I can kind of "get into her head" I may be able to handle the situation a little better next time.

My friend and I are both divorced and have been each others' support group throughout. I'm going to make a long story short; she started flirting heavily with a guy that I was originally talking to at the bar. Now, I just let it be since I was not that interested in the guy; but what causes someone to do this? Insecurity? Just interested in your opinions.