"Hoarding disorder" refers to a mental health condition where a person has a strong desire to keep and accumulate belongings, often to the point where it becomes excessive and affects their daily life. Full definition
One of the worst aspects of it is that people with hoarding disorder often don't see it as a problem. (americanshihtzuclub.org)
But a paralyzing inability to throw away unnecessary possessions can belie a potentially debilitating mental condition known as hoarding disorder. (health.ucsd.edu)
The paradoxical world of hoarding disorder has some answers (newscientist.com)