Hoarders don't hang onto things just for the heck of it. Most of the time, they see significant value in items that the rest of us toss out without a second thought. Sometimes, hoarders view objects as having artistic merit: Driftwood, buttons, greeting cards. Or they save items that bring back happy memories -- even if that item is just a fast food restaurant cup.
Although collecting or messiness is certainly understandable in moderation, hoarders find significance in virtually anything they encounter, resulting in a home full of more odds and ends than anyone could possibly need. Some are unable to discard even obvious trash or garbage.
In addition, they have difficulty making decisions or organizing their belongings, so their stuff just accumulates in piles. It's easy to see how hoarding behavior can quickly spiral out of control.
When confronted with suspicions of hoarding, however, people will often emphasize that the collection of such objects helps them feel safer in their own home. Yet hoarders are also often embarrassed about the state of their home and become antisocial.