Why is the "national debt" a problem when private sector debt is seen as perfectly ok?

[Dovahkiin]

I often wonder why people act like the "national debt" is a massive problem (it isn't, I'd hardly call it a "debt" in any sense of the word.) but pretend private sector debt isn't a problem, despite the evidence showing that the private sector doesn't handle taking on to much debt well at all..
A paper here touches on many things, but I want to point out something:
http://www.levyinstitute.org/pubs/sa_3_16.pdf
Refer to figure 7, figure 9, 20a/20b.