According to experts, there's a scientific explanation behind the gratification we feel when we watch things get unboxed; it activates areas of the brain that involved with "mimetic desire," a subconcious phenomenon during which we imitate the desires of others. An example of this that might sound more familiar to you is when people all start coveting an accessory, let's say, oversized pearl hair clips or puffy headbands. Everyone's desire to own these items is only creating more desire and demand for them.
There's also an element of curiosity; people want to know what's inside the box just as much as the person opening it. We know we do.