As for keeping stuff, that's a personal thing I think. Some people like to keep reminders I think and some don't.
I'm a throw everything out person, mementos, gifts, bedding, cookware - everything but I know one ex of mine kept everything and drove off down the road with it. A lot of the stuff I had paid for, including some furniture, gifts etc. He apparently had no problem with that, but he's an ass so go figure lol (not an addict).
People are different, so I guess your ex is one of those people. Also, alcoholics aren't known for being happy and upbeat (in their heads), alcoholism has a serious impact on the brain and it is a depressant. You might want to read some of the threads in the Newcomers to recovery and Alcoholism threads to see how messed up thinking can get and how these folks try to work through that when seeking recovery.
Perhaps your ex likes to wallow in his sadness?