I think this is because humans are afraid of making themselves vulnerable. There are a certain group of people who express as little emotion as possible. They are afraid of showing weakness because they are afraid of getting hurt. So they don't talk about how they really feel, not even to the people they love the most. They never take that extra step. They don't give out affection nearly enough to those they care about, thinking they don't need to. They bottle up their sadness and sometimes their anger too until it builds up inside of them and comes out in destructive ways. They don't tell other people what they want from them, so they never get those things.
Maybe not all of us are this bad. Maybe we only do it a little bit. It's still a mistake, though, each and every time we are less than honest.
I suppose there are people who show too much emotion or show it too quickly. I find that holding back too much is the more common mistake. If it isn't, then it's the more devastating. Regret for what you haven't done has a special potency that regret for what you have done doesn't quite have.
So instead of feeling that regret (and maybe some guilt for not telling the truth), why can't we just say how we feel? It takes some risk, of course, but sometimes the best things in life require risk. Sometimes those things will set you free.