Im having problems understand my boyfriend!!?

so im dating this guy name christian hes a great guy, very sweet hes a great boyfriend...lately he doesn't wanna open up if we talk about things and i express how i feel he lies and says hes fine....when i know hes not and i can tell by his face and reaction hes always quite and thinks alot and it concerns me he was my best friend now he is my boyfriend ..before we dated we talked about everything things that were private in his life and mine...we started dating and he said he is comfortable talking to me about things but when it comes to expressing feelings he doesn't do a good job..im trying to understand ....why he doesn't i wont judge him or anything...but when we are together we are so loving and we are so cute together and everyone thinks we are in love he says hes in love but sometimes i wonder what he thinks...please help me i need to know why he lies to me when he says hes okay