I work in a fabric store. 'nuff said, lol!
I keep telling myself to either finish what I already have going before buying more fabric, or find a pattern, browse my stash FIRST, and then hit the stores for whatever is missing, but I tend to see a certain fabric and tell myself I love it so much that when I actually find a project that I could use that fabric for (or finish a current one so I have an excuse to start another) there's no way I will be able to find it... so I buy it, lol!
OR, I buy fabrics that go in a set, and I don't want to use just one of the fabrics from that collection in a project, because they were bought to go together, and the pieces that I bought to go together don't necessarily all 'fit' for the project at hand.