Can one use the word "memorial" (noun or adjective) without the negative/sad connotation of commemoration of the dead?
I would say that if you are commemorating a person, dog, or any other physical thing, then "memorial" indicates that the thing no longer exists, or—at a minimum—cannot enjoy the commemoration for some reason. To invite someone to their own memorial would be very bad taste!
Further, if you are commemorating an event, then "memorial" implies (to me) that the event itself was some kind of tragedy.
I would definitely use "commemoration" if I wished to avoid the sad connotations.
How about memento?
an object or item that serves to remind one of a person, past event, etc.; keepsake; souvenir.
Presupposition: A memorial is a device for preserving memories.
Hinging on the above requirement, I would say that, by definition, it does have to be negative.
- If it's a memory, then it no longer exists.
- If it's a memory worth preserving, then it's a positive memory.
- Therefore, since it's a positive memory that no longer exists: it is negative.