You may think that girl is beautiful, and that she knows it so why should you tell her? She hears it all the time. She wouldn't pay any attention to it. But maybe she actually never gets told that except by her closest friends if even, and because of that, she doesn't think she really is pretty. Maybe by telling her that, it will make her day. Maybe she won't cry herself to sleep tonight because "no one thinks she's pretty".
Let's say this boy has a wonderful voice and loves to sing. But maybe he's super shy and doesn't like to sing in front of people, or at least, that how he was a few years ago. Maybe now he's come out of his shell a little and would love to sing for people once in awhile if someone asks, but he's too modest to suggest it. So now he's left thinking that people don't actually like his voice. So when you ask that other guy to sing all the time, but never ask him, he feels like no one likes his voice, and he's not good. So now he's left thinking he should give up on his dream to be a professional singer because he'd never make it.
So I ask you to not avoid saying anything, but to actually have the guts to go up to someone and compliment them on something. Even if you don't really know them. Especially if you don't really know them.
