Maybe our culture isn't so 'best friend friendly' now. I read an article a couple of years ago - around this time of year because it was talking about new year resolutions - and it said that if you have known your best friends for more than 6 months you should ditch them and get some new best friends so that your life wouldn't become 'boring' and 'stuck in a rut'.

Couldn't believe what I was reading - after 6 months, I'd be thinking that someone was moving from being an 'acquaintance' to a 'friend'. And out of my 'best' friends, the newest friend is someone I have known for 9 years!
I guess I would define a 'best' friend as someone you can guarantee will be there for you and whom you can tell anything to without worrying about being judged/rejected or gossiped about. My best friends are all girls I've known for a long time (1 x 32 years, 2 x 27 years and 1 x 9 years) I see one every week or so, the other every couple of months and the other two hardly at all (one lives in NZ now!) but I know I can trust them and they would support me in any way they could when I needed them. Of course, its a two-way thing.
I have other friends I meet with for socialising but wouldn't necessarily regard them as best friends - they are just folk I get on well with and enjoy hanging out with. Maybe at some point one of them will turn into another 'best' friend, but I don't think that's something you can plan or make happen.
If you want more/closer friends, I'd suggest starting by building on the friendships you have - think of someone you might like to know better and invite them round for coffee/out for a day shopping.