He doesn't realize how terribly sad she is, and what she is sad over. He assumes her to be fine about the marriage.
"...Which, too much minded by herself alone,
May be put from her by society." Paris believes that if Juliet had someone to be around her, she would be happy.
[when Juliet enters] "Happily met, my lady and my wife." He immediately assumes she is happy to see him; although that is subjective depending on how one reads the line.