I've pondered the same question myself, and my conclusion is that they do it for one of three reasons:
1) They really truly believe it (however misguided they might or might not be)
2) They are trying to stave off personal guilt that would come from telling you what they really think.
3) They are trying to stave off social awkwardness that would come from others based on having told you what they really think.

As others have said, and I will re-iterate, you can't perceive inadequacy purely because you don't have a partner. It's not healthy and it's not rational. You don't have to conciously "sell yourself", but if you behave the way you seem to be, you're selling everyone else. Why make your job harder?

One thing that I've heard before that some of the respondants have almost mentioned here is that if a woman is tending towards being attracted to you, and you start unloading all this stuff, you're pissing on her judgement. Not good.

I have more, but I can't say it coherently right now.

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