Friendship morphology?

I'm a busy girl this week! Didn't do much on Monday, but yesterday the kids and I got to have lunch with April to chat and for me to deliver her Thirty-one stuff. Today I Fuss has a play date with her best friend, Grace and I get the chance to chat with my friend Kat. Tonight Fuss has dance. Tomorrow, we have Bible study and Friday... we don't have plans, but I'm expecting my order from Thirty-one to arrive, so I'll be spending some time sorting and working out the delivery schedule. I am SO psyched. Saturday we're going to the City of Halloween/Trick or Treat event and having a garage sale in the AM. I know Fuss is excited to T or T.

I was having a conversation with my husband last night about adult friendships. How i always wanted a friend that would be my best friend until we're old and grey and we move into an ALF together or something. I've always felt like the odd one out. The third wheel. It's frustrating. In high school, I was part of 2 close-knit groups, but I was always the odd one when there were pairs. (Freshman year I was part of a foursome that did everything together. J and T were BFFs and H was my BFF, but she had a BFF from where she had lived before. And then she got a boyfriend and I was squeezed out. We remained friends, but weren't as close. Sophomore year it was my friends Liz and Cherry - we've all been friends for eons, but THEY are best friends and I'm just along for the ride. You get the idea/see the pattern.)

I feel like my closest girlfriends around here don't see our friendship as important as I do. And I feel left out. I and I feel stuck. I feel like my lot has already been cast - my adult best friend is K and that's who I'm stuck with, for better or for worse. My husband says that's not so. And if I want to pursue a friendship with Kat, that I should.

Is it immature to continue to label people "best friend" as an adult? And how does that work anyway? I mean, when you're on the playground as a kid, it's easy to say "do you wanna be my friend?" or "my best friend is ______" and it's done, but how does that happen when you're 30?