I also don't intentionally just build stash - it just happens. The fabric talks to me, softly calls my name or jumps out at me 'take me home!'.
I usually have a good idea of what sort of project I will put together with the fabrics I select.
The stash happens because there are *SEW MANY* projects ahead of my production! And also because I always buy more than I need for a specific pattern. And then, also because I see a grouping of fabrics, and take 2 or 3 yards (or metres) for each one. That is how my stash happened!
Hmmm ... just a wee bit out of control ... LOL!