Guys build walls to protect themselves.
When you build a wall, you simple protect your mind space and make sure that you stay in control.
If your partner does not want to talk about his feelings and "open his heart" to you, he might simply have other priorities right now.
He might be busy with work issues or simply have other worries.
He won't open up because stirring emotions and feelings simply require energy, time and space.
You can't force a guy to open up.
That's precisely the kind of attitude which will turn him off.
Relationships are about love but about power as well.
If your partner opens up all his feelings to you, he simply drops all protections and establishes himself in a new territory.
If he wants to keep his "edge", a bit of mystery maintains the "tension" and attraction.
If he does not open up, it can as well be that he is afraid that what he gives you might be used against him at a later stage.
That's always a risk in relationships.
Opening / Not opening ... At the end, it is a bit of a power game.
It has not much to do with love.
It's more of a natural reflex and defense mechanism.
If you feel his attitude is hurting your relationship the best thing to do is not to push him, it is to give him space.
That way, he'll feel more free to open up when he is ready.
Of course, the next is: “So, what can you do about it?”
The answer is simple: when he does open up, give him space to share if he wants to. Make sure you listen, don’t try to solve what he faces, don’t push, don’t demand or get upset.
A man will find much easier to share his feeling or impressions with you if he knows that he is in an emotionally safe environment.
In a way, you have to be very diplomat and mature and if something bothers you in what he says, never attack back. Ask for more details instead. Use expressions like: “Can you tell more about this? I want to understand exactly how you feel”
If you stay within the context of empowering each other and creating relationship synergy, openness and transparence will tend to happen naturally.