I think it probably stems from the days when knitting was less a hobby, and more of a necessity to keep the family in socks and whatnot. So a bit of a cultural/social hangover from those days, likely with a feminist aspect too (women's time/skills not being highly valued). I certainly observed that in my grandmothers, and to some extent in my mother. I have no problem knitting for myself, though I do enjoy making items for people I care about. I am fussy about who is 'knitworthy' however!

I never use it. If I make something I will decide if it is for me or someone else. Why should someone have a right to my hobby?

Love the mitts. I first saw it donkey's yonks ago as the title of a Ravelry group. To me, it insinuates that if you're not knitting for someone else, you're a pretty rotten person. I knit as a hobby. Knitting for other people is not something I enjoy. When I'm asked if I'll knit something for someone else, I offer to teach them to knit. I've never been taken up on the offer.