I'm thinking of keeping a present not meant for me
I think that's cruel. Why would you do that?
I know it's cruel to keep it, but it's also cruel to leave me with such an enticing present and expect me to pass it along.
Clearly a mistake was made when someone gave you the gift that was meant for someone else, but that does not make it okay for you to keep it. As much as you like the gift, you are depriving it's true recipient, and also upsetting the people who meant to give it in the first place.
Why shouldn't I keep it for myself? I deserve nice things.
The person that gave you this present didn't intend for you to have it and keeping it would be the same as theft. It doesn't belong to you so give it to the intended recipient.