The only person I share my makeup with is my mum, and that's only because she always asks me if she can borrow whatever she'd like to wear--usually eyeshadow. And she'll usually ask me to apply it as well, since my mum isn't exactly what you'd call 'girly'. Neither am I for that matter; I got into makeup and makeup artistry last summer but before that? I had one lipgloss from Walgreen's, and it was ultramarine blue with glitter.
As far as my friends are concerned...my best friend is dragged to the Galleria every summer without fail, and if she sees something she likes even moderately, I'll buy it for her. She's attending college and paying for it herself and I'm still stuck at home with no expenses so I buy her stuff for her. She's building up a pretty good makeup collection, too.
But as far as actually lending my stuff out? Hell fucking no. When I was a junior in high school, I lent my purple Dior Plastic Shine to a girl in my art classes--and keep in mind that by my junior year all but three of my classes were art classes--and even though I saw her EVERY GODDAMN DAY, she was always 'forgetting' to bring it back to me. When I finally got it back, there was maybe ten percent left.
...I made that bitch buy me a new Plastic Shine, goddammit.
And plus, like everyone has stated before me, there's the hygenic reasons.