My neighbor is a hoarder. Many animals hoard by nature, and they say human hoarding may be related to that but there is substantial evidence lacking there.
My sister and her husband are both hoarders of things.  they love to buy in bulk and just continue to keep everything they buy therefore their house is full of stuff
That kind of behavior has been associated with health risks , economic burdens, and adverse affects on loved ones. It's unfortunate.
It is very unfortunate.  We have helped them a few times clean their house but it just gets full again so it seems somewhat pointless
It's not easy for us to understand, but hoarders really do have a feeling of attachment to these items they collect. 
yes, it is hard but I do try to talk to her and figure out the underlying issue.  I am the complete opposite end of the spectrum and I like to throw everything away and have things neat and orderly
I am the same. The most you can do for her is to make sure she is safe, and that her house doesn't become a fire or health hazard.
yes, this is true.  One thing I am happy about is that she doesn't hoard animals