Comment by z33k

I'm not an astronomer, but my intuition is this: When the source of the light is moving towards the observer, each successive photon emission happens from a position closer to the observer than the previous photon. Hence, from the observer's perspective, the time between photons is reduced, meaning more photons are observed in a given time, and the brightness is increased. When we observe a galaxy that is rotating opposite to us, not only is the source of the light moving closer to us, but we are also moving closer to it.