I do this. If I read a library book that I really like, I always end up buying it. I want it to be part of my personal library so I can return to it again and again.
Same here -- and lately also with audiobooks. I'll start to listen to something and then very quickly decide, oooh, this is so good I'm definitely going to want to return to this, and maybe also in a different medium ... so I end up hunting down a print copy in addition to the audio, so as to have them both in my library.
it's a rare gift among book readers to be able to abandon a book you don't like. Most people would force themselves to finish. I am in between. It has to be super bad to mark as dnf. Usually it goes in the on hold pile.