Kate Upton's new boyfriend was initially reluctant to date her.
Former 'Dancing With the Stars' professional Maksim Chmerkovskiy, 33, reportedly feared the 21-year-old model was too young for him but fell in love with her after they became close friends.
A source told People magazine: ''(He was) hesitant to start things with Upton because she was so young.''
But the insider added: ''They have a lot of fun. She's very mature for 21.''
The couple started dating after being introduced by mutual friends six months ago.
The source said: ''Along the way, it turned into something else. They just clicked. They are very serious. They have a lot of chemistry.''
The couple were photographed looking relaxed and happy as they strolled through New York's West Village hand-in-hand last month just weeks after Kate insisted they were ''just good friends''.
Kate, who insisted last month that she doesn't ''put relationships as a priority'', recently finished filming her first major film role in New York alongside Cameron Diaz and Leslie Mann in 'The Other Woman.'