I vote all of the above.
1. I need it for a project;
2. I like it and I will find a project to use it on;
3. I have no clue what I'm going to make, but I just have to buy it because I see it. Oh and number 4 gets me everytime "YARN SALE". I am so addicted to crocheting and because of that I am addicted to buying yarn.
Here is a funny story....Last month I was on my way to South Carolina to visit my son and granddaughter, two exits before we need to get off, I say a hobbylobby store sign. I wanted to see if all the hobbylobby store carry the same sort of yarn (well that was my excuse for getting off at that exit), and of course I have to buy something, so I end up spending $30 in yarn...hubby was not happy, but oh well, he'll get over it.