Being a bit more serious..... My stash began when I first started quilting. As I toured a fabric store I would see fabric that made me think of a sister, a grandchild, my dear friend.... and I bought. I always buy more than I think I will need and over the years I've been glad. Sometimes the fabric excites me so much I buy it (maybe for a blanket for DH and me) maybe to put it away and wait for the "right reason" or the "time to quilt", perhaps for some unexpected reason. I have a huge music motif collection of fabric because I have grandsons who go to music schools and I also make bus quilts for the on-tour bus.
I think "stash happens" is the best explanation. You and I are not the same and what happens to you is different than what happens to me. Space is a consideration. Cooperative husbands (some DHs grumble, some discover fabric for you) are important. Some quilters are very much into one sort of "feel" with their quilts and their stash reflects that quality. I can't imagine having bolts of fabric myself... my attention span is too short for me to want to make more than one or two quilts using a certain fabric. Also, I love digging through my fabrics (I have an 8'x4'x14"deep cabinet about 2/3 full plus plastic drawers for scraps) and it's enough for me. Yes, sometimes I have to go out and find fabric that will complete/complement what I'm doing, but usually not. My stash is totally mixed, with some panels, some florals, etc., but no plaids. I just don't "do" plaids. Hope this adds to the discussion.