Natasha might have a slight weakness for the contact. As a human herself, she rarely allows people the familiarity of too much contact, not unless there's a purpose to it. This, though... Her identity is more than safe at the moment and her feline side is much more susceptible to giving into a little indulgence, especially around someone she trusts - to an extent. He may not know her secrets, but she knows enough about him to not be wary right now.

Besides, he's petting her and it feels good, so she sees little harm in encouraging it. When he continues, she ends up all but sprawled in boneless enjoyment on the carpet in front of where he crouches.