Someone who calls you their best friend after one week is either very lucky or very practiced. Take it slow.

Instant intensity says more about the other person than about the bond. Closeness declared in a week has not been earned by anything, so it can be withdrawn just as quickly, and some people move at that speed with everyone they meet. It feels wonderful and it is not yet information. The quality of it shows up around month six.

Enjoy the warmth without reorganizing your life around it. Keep your other friendships running at full strength. Let this one prove itself over a season, and watch whether the intensity survives the first inconvenience.