He has a secret? Please help!?

Last night at a school dance my best friend (a guy, I'm a girl) told me he had a secret he hasn't told me for 2 months, almost with a sly smile. He loves playing jokes on me and making me freak, but idk what the secret could be! When I asked him if he'd told anyone else, he said a few people. This actually kinda hurts, cause I'm usually the first person to know about everything going on in his life. When I asked him if I will ever find out, he just said idk and tried to change the subject. I'm so confused why he didn't tell me before, and why he's refusing to tell me now. Please help!