I think that what makes something good is the feeling you get when you use it or interact with it. A good book is a book that you just can't put down and enjoy reading from cover to cover. It stays with you and leaves you wanting more when you are finished reading. You can lose something and still have it if you have memories of it or don't physically need it to remember what it is or what it stands for.
I think what makes something 'good' depends on who is doing the deciding. Something might be good for one purpose or person, and not good for another. I think it's subjective, like Angelo mentioned in his response. Also, you might need to think about short or long term goals. For instance, my kids might think that getting extra iPad time is "good" right now, but when they are tired, overstimulated and cranky, I might not think it's so 'good'. Also, even though something might be 'good' (e.g., medicine, daily exercise, whatever) for a person, it might not necessarily make them happy.
I would define a good book as one that's still having an impact long after the pages are closed. I don't think you necessarily have to enjoy it, but it serves a purpose, even if that purpose is just to help you get away for a few minutes or hours while reading it.
And again, I agree Angelo. You can physically lose something and still have it -- either through memories, or through relationships that it helped build.
What makes something good is wanting more. A good book leaves you wanting to read more. My favourite books are ones that create sequels or ones that I wish would create sequels. As others have mentioned, having the memories mean you still have something even if you lose it.