I should add, on the same page, there is an excerpt of an interview with his sister Lynn who set Dennis up with a friend for her junior prom. So he at least dated.

More broadly:

"As a youth Dennis was social and outgoing, then in young adulthood he transformed quite rapidly to become more private and anti-social. Hard to pin down exactly when this would have been happening… except that clues from John and Lynn’s interviews suggest perhaps sometime around February 1968 might be central."