Telling someone you care about them would seem to be a pleasant situation. When someone has been an important person in your life for a long time, why is it difficult to say the words? The internet is full of information about how to show love and caring for someone. Books have been written about love languages. It seems it's something that more than a few of us struggle with.
I think it might be because expressing positive emotions makes us vulnerable. When you express caring or love for someone, you make yourself vulnerable to rejection. Maybe not the rejection of you as a person, but perhaps rejection of your words. Sometimes receiving affection can be uncomfortable even when people enjoy it, so they make a joke or tease you, which can be painful.
Perhaps there's also the risk that the person might see your caring as an opportunity to take advantage. Perhaps it feels like you will be one-down in some way? Or maybe you will be seen as a wimp? Yet we go to movies that show authentic expressions of caring, buy cards that express loving sentiments. I think we all need to hear and feel the affection that others have for us.
Expressing anger or negative emotions doesn't create such vulnerability. And most people don't seem to struggle with expressing affection to young children.
In most situations though, the person you care about (friends, family) is not someone who will reject you or use your feelings against you. Perhaps the difficulty is that expressing caring feelings toward another person is something you don't do that often. Perhaps you show your feelings indirectly (buying gifts, teasing, fixing things) and are more comfortable with those forms of caring.
Does it matter? Expressing your feelings can be a form of intimacy that enhances relationships when it's done in the context of a close relationship. Verbalizing your caring is a way of deepening your connection.
Bring to mind someone who is a family member or friend who lives far away. When was the last time you expressed affection to that person (not on a holiday or birthday or other celebratory time)? Just out of nowhere called to say you were thinking about them or sent a card with a handwritten personal message about your feelings for them?
What about people you care about and see every day? How do you express affection for them? Do you tell them they are important to you and why?
What about your spouse? Do you tell him or her what you appreciate? Do you reach out and touch his or her hand just because you care? Do you wink at him or her or otherwise show you care?
Recently a long-time friend took me to lunch. She told me she wanted to tell me how she felt about me. Even before she started, I was near tears. I was touched by her expressing her caring for me. By the time she finished, we were both in tears. She expressed herself openly and authentically and the experience was a meaningful one, and so intimate.
Perhaps expressing affection to someone is a gift. It felt that way to me.