Khadgar42: Actually, as far as I remember you can "lend" your copy to a friend. Someone with blue text told us that here somewhere. They just ask you not play it while you have given it to a friend, if I understood this correctly.
That's really kind of gog actually, but anyway sharing = torrents is illegal as you don't have control on how many copies have been shared.
I think that should be treated more as GOG's opinion on it, what one could use as a guideline. GOG doesn't really hold the IP rights to the games, albeit I don't know what kind of deal they have with the publishers concerning the extra compatibility work and installers GOG creates on top of the actual games.
It is the game publishers, and ultimately the legislation in your country, which decides what you can do with the games for which you have a license.
To me it is morally somewhat ok approach that you'd loan games to people closest to you the same way like you'd loan retail games which have a CD check. If you loan it, you shouldn't play it yourself, nor loan it to many people at the same time. Yet, I am unsure if even that is really legal with digital games.
I would also say that generally it is a good idea not to loan the actual installers, but install the game yourself to someone's PC if they want to play it. Just so that it doesn't become too easy for him/her to also "loan" (=share) it to all his friends, who share the installer to all their friends etc. etc. etc. Since you are the license holder to the game (installer), you should also remain as the only one who controls its usage, the best you can realistically. Don't wittingly share that control also to others, by providing the installers.
I personally don't want to share my games at all outside my household. Not much reason either though, I don't seem to have real-life friends who'd be hot after mostly old PC game classics anyway.