I Always Get Asked Why Im So Quiet

This has become a problem lately. Apparently I don't express myself verbally enough. It has made my partner very frustrated with me. I'm not a talkative person, never was. He knew that about me from the beginning. But now he needs me to talk and say what I'm feeling and thinking, as much as possible. For him that's so easy, he has a hard time understanding why it's not easy for me.