Hidden Password

My boyfriend of almost 2 years has refused to give me his password on multiple occasions. I don't know why he wouldn't give it to me. I honestly it feel as though I should know it although I don't plan on going through his private business. No matter how many times I beg him to give it to me, he refuses. He justifies this by allowing me to go through his chats on what's app, although he always ends his chats and the only chats on what's app that I can even read are normally between himself and I or a group chat. 😐
Should I be worried ? 
Is he cheating ?