why do guys have to hide their true feelings?

Not all the guys hide their true feelings. It depends on their own characters. Maybe you have to ask them directly or indirectly about their feelings if you want to know the answers of your doubt.

I think it is the way guys are raised differently than girls. They are taught not to cry or express feelings in public, etc... They are more reserved with their emotions which makes them harder to open up and talk to you. This is not true with all guys, and some are just fine. So it just depends on what guy you happen to be talking to.