An appoggiatura is a type of ornamental note that clashes with the melody just enough to create a dissonant sound. "This generates tension in the listener," said Martin Guhn, a psychologist at the University of British Columbia who co-wrote a 2007 study on the subject. "When the notes return to the anticipated melody, the tension resolves, and it feels good."
"Someone Like You," which Adele wrote with Dan Wilson, is sprinkled with ornamental notes similar to appoggiaturas.
The science of music is just fascinating. I had never heard of this before, but now I'm going to be listening for it everywhere.