Do you hear the pulsing sounds? I'm hearing pulses in the delta to theta range (frequencies ranging from 1-7 Hz). Research shows that when our brains go from an "everyday life" beta wave state (varying from ~14Hz-30Hz), usually associated with higher thinking, concentration, worry, anxiety, and agitation, to a slower alpha wave state (8-13 Hz) or theta brain wave state (4-7Hz), we have a greater sense of relaxation, happiness, and joy. On a molecular level, its been shown that greater levels of serotonin are released into the synapse during the alpha state (and I'm guessing also in theta). Are you using a selective serotonin reuptake inhibitor (Celexa, Prozac, Paxil, Zoloft etc) to help you with your depression? Similarly, these type of drugs act to keep serotonin in your synapses, thereby helping to treat your depression.
But how does this relate back to the baby video? When we listen to,
"Biggie, Biggie, Biggie, can't you see..."
we're hearing rhythmic frequencies around 5-6Hz (in theta) and a various mix of other alpha frequencies. To register those sounds as information, your brain needs to fire neurons at that rate (and as discussed, these frequencies are shown to have positive effects). Don't you love being absorbed in your favourite songs? The more we focus on the sound, the greater amount of our brain's resources begin to fire at those rhythmic frequencies, and thus we get greater enjoyment from the music.
So is this why the baby is loving Biggy so much? And is this why we love music? Maybe. Albeit, its probably much more complicated. My request to you - maximize the experience you can get from music - be like the baby. Stop doing other things while listening to music, be present with it, stop thinking, and get absorbed in the music (i.e. close your eyes and put some headphones on). You'll be happier and more fulfilled for it...and well, that is just being mindful, isn't it.