What's the reason behind cabal (dependency) hell?

Question

How does dependency hell happen in Cabal-install?

I read the following at Cabal/Survival - HaskellWiki:

1. What is the difficulty caused by C

Answer 1

One way dependency hell happens is when several different projects you are working on interfere. While a single project has a solvable constraint set, two different project might not and since they're sharing a single package DB, there will be trouble. This variant of dependency hell is solved by cabal sandboxes.