This is Us is quite a clever title because it could be read, and perhaps it is subliminally registered as This is U.S. Anyway, it is about a number of characters all in a different state of dilemma or potentially life-changing time of decision. Viewers can readily identify with one of these characters or empathise because they know someone closely who resembles on of the characters.
A weight problem, a change of loving partner, a work crisis, these are all situations that viewers are very likely to have come across. Then the dialogue is often bold and witty, so it's an enjoyable show to watch. And it's on every week.