Say what you mean and mean what you say... Also known in some circles as honesty. I believe in honesty. I appreciate honesty. I think most people do. But honesty is not always easy to come by and it isn't always easy to acheive.
As much as I believe in honesty, I also believe that if you can't muster the courage to say exactly what you mean, you should say nothing at all. Save the conversation for a later date when you are more sure of yourself and less worried about the other person's reaction.
The pitfall in many relationships, freindships or otherwise, is that sometimes we talk just to fill space. And sometimes, in order to keep up momentum, we say things we don't really mean. Above all else, we often say what the other person wants to hear to get what we want, to avoid an argument or to find our way out of a conversation. Everybody does it.
There are some things we should never say... unless we mean it.
For example, "I LOVE YOU." Over the years, I have become much more liberal with my use of those three words. I tell friends and family often that I love them. The tricky thing about those words is that the interpretation on the receiving end varies based on that person's feelings for you. If you have never had a conversation with your friend and confirmed that you are just friends, you probably shouldn't say it. Other phrases that fall into the same category as "I love you" might be "you mean a lot to me" or "you matter more to me than..." and most of all "I need you."
There are many other examples including "you look great," "I enjoy spending time with your mother," "I would take a bullet for you," etc.
Think before you speak. If you honestly mean it, then say it. Put it out there. At all costs. But if you are unsure, wait. Or choose your words wisely.
Don't say it unless you mean it.