The first time he meets her, she dresses him up like a kitty and invites him to a tea-party. He accepts in the name of irony - or so he tells himself. He also tells himself that the amount of fun he had at the tea party was also due to irony, and not to Nepeta's infectious mood and good conversation.
The second tea party invitation, which was also accepted 'in the name of irony', is a little different. This time, she didn't dress him up - and she made him apple juice. Just for him, she said, and he could have sworn she winked.
By the third invitation he can no longer lie to himself. Nepeta is the most adorable, fierce and thoughtful troll he's ever met. Terezi is cool, as a best friend, but Nep makes him. Well. Makes him feel different. He accepts the third invitation, and spends an hour deciding what to wear.
On the day of his thirty-second tea party, he asks her to be his matesprit in a way she appreciates ('will you make my otp canon?' he asks, and she pounces on him and refuses to let go for half an hour). She accepts.
