There's one thing in life that I don't let people get away with, and that's talking down about themselves. No, I don't punish them or chastise them, but I always correct them I won't let a self-deprecating comment go by without countering it with some sort of positive reinforcement. This annoys my step-children to no end they always say "But I was just joking," or "I didn't really mean it." And that's probably true they didn't really mean it. But I don't let the comment slide for several reasons.
First of all, I know the power of negative self-talk. Saying bad things about ourselves can lead us to believe them, even if we start out "just joking." What happens if we make a negative comment about ourselves and no one disagrees? There's a part of our minds that will tell us "Hey no one's arguing! Maybe they agree with the comment!" This seems to be the case especially with young people who are in their "developmental years" (though aren't we all always in those years?). Especially in our culture, though, we're taught to learn things through indirect methods ("Ask Rob if he likes Sally"), and we come to expect to learn things about ourselves more through what other people say or don't say than through other more direct means.
Secondly, I see such comments as an opportunity for encouragement. As an adult, I am a role model. Period. I can choose to accept that role, or I can choose to reject it. As a role model, it's important to me to provide young people in my life with a healthy, encouraging, helpful way of being, and most of what people learn from me or of me has to do with what I say and what I do.
But there's also the more subtle side: what do I let slide? What do I let go by without comment or action? Even among people my own age, I know that it's important to encourage whenever I can, and not to let an opportunity to encourage go by. I don't know if that particular person needs encouragement at the moment yes, they may be fishing for a compliment, but they also may be in a very needy time of their lives. I'm not concerned with judging why a person is needy, but I do want to recognize it when a person is needy.
(By the way, this can only go so far after the third or fourth repetition of the same negative comment, I'm much more likely to tell a person to knock it off than to encourage. There's a certain point when the concept of hard love kicks in.)
Third, I want anyone else who might have heard the comment to know that at least one person finds such comments to be completely inappropriate. I don't want to let someone talk themselves down and have someone else think that it's normal or acceptable to do so. Someone has to say something, and this is another role that I'm willing to assume. If our 13-year-old hears her 15-year-old sister make a comment insulting her own physique, for example, and no one says anything about it, she just may find the same or similar flaws in herself and start to worry about them. If she hears someone tell her sister that she shouldn't make the comment because she's fine just the way she is, she still may find the same "flaws," but she also may be much more accepting of them just the way she is.
Life is about other people loving and helping and encouraging them. Doing that will give us meaning and fulfillment in life, and neglecting it will harm us. We have to be aware, though, that the only people who can counter another's self-sabotage through deprecating self-talk are those who hear the talk. We have to counter it--I know that I would have been spared years of negative self-image if anyone had bothered to counter my negative ideas about myself when I was younger. Now that I'm old enough to do so for others, I counter it every chance I get.