I Love to Love and Be Loved

only recently I came to learn how hard it is to love someone and not be able to say those words to the person.
"I love you" its on the tip of my tongue and yet I cant say it out load, it's like I have a rock in my throat. The worst part is that I am an extremely honest and direct person.
If I say it too soon or if its single sided it will be the end of things. But why? loving someone is a beautiful thing and yet people are so frightened by it when its not mutual.