I usually paint with watercolour, for the simple reason that a small box of watercolour paints is easy to carry when travelling abroard or to a group drawing session. And the paint is quick drying so I don't have to carry wet paintings around. All the painting is done in one session, on location or with the model in front of me - I very rarely work on a painting without the subject on front of me, so I have learnt to work quickly and spontaneously.
I also love the way watercolour paint has a life of its own when it's wet, running in unpredictable ways, how the fresh colours merge, and how the luminosity of the white paper shows through. I sometimes combine painting with other media, such as a wax resist, or inktense pencils which give me water-soluble lines. The magic is that there is always more to learn and explore with watercolours, mistakes to be made, and I accept that I throw away as many paintings as I am pleased with!