I just have to be prepared. If I wake up in the morning and decide I want to start a project, I don't want to have to spend half the day looking for fabric to start it.
My stash also grows because I choose fabric to start a project, then one of the fabrics I have chosen will send me back to shop for more fabric to add another level of color or print. Which also leads to purchasing additional fabric. It is necessary to be flexible, especially as your project evolves and grows.
We share lots of fabrics in our quilt guild -- it offers each of us more options. Often someone else will have that fabric that we would not give a second glance to, but our block needs that little something to make it complete.
And, buying fabric just makes me feel good!