Hmm most of the time it's hearing an interesting sound in a new track! I love creating my own sounds firstly because I know no-one else will have used em, unlike using presets that everyone has.
I also like the control you have over creating your own synths. When I used to mainly use presets I used to spend too much time going through a bunch of vsts looking for something that sounded close enough to what I was hearing in my head. Now, when I'm hearing a certain sound I just make it, and I can manipulate it to the way I need to!