Spencer had spent years pinning over JJ but she was never interested in him so he moved on. He met Kenzie at a book store she was doing a book signing and he hadn't read any of her books. She actually approached him and hit it off. Over that year JJ was having trouble dating and wondered if Spencer would still have feelings for her. So she started to flirt with him he never flirted back. See he hadn't told the team he had a girlfriend as he wanted to keep his professional and personal life separate. He was getting sick and tired of JJ's flirting so when Rossi threw a dinner party he brought his girl with him JJ was late so she missed the introduction of who this mystery woman at the party was. She was looking at Rossi's beautiful mansion when JJ came up to her. JJ: Who are you? Spencer was within ear shot talking secretly with Morgan about engagement rings. She could hear the judgement and jealousy in JJ's tone