Why is it so hard to show our feelings?

This is something I have been asking myself during ages.
It's amazing how things can go wrong because we don't show our feelings to the people around us. I don't only mean saying our feelings directly but showing them in any way. For example, to show love you don't need to say "I love you", you can kiss the person you love, hug that person or just look at the other's eyes. It seems so simple while I type here, but the truth is that it is not so simple for everyone. Why? I don't know.
But it is not only about love. It is about every feeling. Sometimes we miss a person like mad but we don't find the moment or we are not brave enough to take the phone and just say: "I miss you".
I think the world, or at least the personal relationships, could be better if we show our feelings.
Don't you know someone that has lost his/her love because they don't say "I love you" to the other? Don't you know someone that regrets not saying "I'll miss you" before someone died? Don't you know someone who regrets not holding someone's hand just because she/he "didn't find the right moment?
I think we should say these kind of things every day, like if tomorrow we weren't going to have the chance to do it.
Life is too short to be hiding feelings, and more when we can lose the opportunity to show them and never have it again.