My grandmother definitely had hoarding tendencies, but she was at least a "neat" hoarder. Closets, basement, attic and garage were full of perfectly good stuff, all still packaged. The rest of the house was pristine. Her thing was buying stuff on sale.....if it was on sale, it was irresistible. But she just couldn't give the stuff up, for some reason. For instance, say someone she knew was getting married and had indicated that she wanted a pressure cooker......she'd have three brand-new pressure cookers in the closet still in their boxes, but she'd go out and buy another pressure cooker for the bride.
She would never have considered buying anything used; it always had to be new and it HAD to be on sale. She WAS very generous to family, though, and if there was something you wanted, she would give it to you.....and then go and buy another one.
I can't figure out why she was the way she was. She grew up in relative comfort, had a pampered childhood and as far as I know, didn't have any great tragedies in her life. My grandfather was a devoted husband and worked hard for her.
Some of it must have been genetic as her older sister was a hoarder too. My great-aunt had every phone book that had been published in her city during her life, every broken comb, every piece of paperwork, etc., all neatly packed away in boxes, stacked neatly in rooms and hallways and covered with lovely hand-made doilies that she took off every few weeks, washed, starched and ironed neatly and put back.
I carefully fight any hint of hoarder mentality that I see in myself. I see a bit of it in one of my sisters.