Why do guys hide their feelings?

If there’s one thing that bothers me about guys, it’s when they simply cover up their feelings and keep me guessing. Guys and girls differ in an innumerable number of ways but why can’t feelings be shared just as easily as girls can share their feelings? It makes it really hard for me to put trust into someone when I’m always guessing how they feel. If you answer the first part of this question, maybe you can help me with the second part of the question: how do you get a guy to open up? I have a hard time coming out and just telling a guy something, but I need to start doing that if I want any sort of luck in future relationships.