David Hume's "Enquiry Concerning Human Understanding"

Hello, I've just started reading David Hume's "Enquiry Concerning Human Understanding" and I'm trying to figure out why Hume argues (look in part 1 of Section 4) that knowledge of "causes & effects" can't be gained through reason alone?  Why can't it be?  I think it's logical that reason works alone sufficiently.  Please explain without using Cliff notes or Spark notes ...just plain, simple language would be appreciated.