Why is he acting like this?GUYS AND GIRLS?

ok. I'm a 12 yr old gir nnnnnnnnnl and my friend is a boy whos the same age. We are the BESTEST friends of ALL TIME!!!!!!! ^ ^. Now he's acting all weird since he turned turned 12. We used to be "I can beat you, No you can't,yes I can... and so on but now he's all...different. he used to like a lot of people with him but now he just wants to hang out with me by ourselves. if im sitting next to him, he asks if i want to lean on his chest and i'm just wondering why he's just so suddeenly changed. Anyone got any ideas cuz my minds a blank