Watercolor is a partnership between the artist and the watercolor. It is very different from oils in that regard. In oils, the painter is truly the master. Oils will do exactly what the painter tells them to do and nothing more. Try to mold watercolor entirely to your will and it will rebel. It must have its say if its true beauty is to be revealed. That's not to say the painter can't participate in the process through the application of sound principals (drawing, composition, shape, value, texture, color, etc.). However, the unexpected should be the norm. Watercolor is just colored water and by definition very fluid. It chases any moisture on the paper and will produce random effects totally unexpected by the painter. Such results are often unsettling to the novice watercolorist and produce an "urge to purge". The painter becomes panicky or at least overly concerned with "correcting" what they see as a blemish on their preconceived idea of what their painting should look like. More often than not matters are made worse by attempting corrections. I have learned to live with these surprises. Just press on. As the painting develops, these expressions by the watercolor usually become insignificant or make wonderful additions to the composition.