why?

Someone once told me, that even if you're in a room full of people, you can still feel lonely. I do feel cheered up when I'm with my friends, with my family. But recently, i've become so sad. This isn't like me.
I feel so lonely that I just want to curl up in a ball and cry. I want someone beside to share my laughter, to share my tears, someone to ease my loneliness, someone who can cheer me up and be there for me when I need a shoulder to lean on the most. I want someone to share my life with. Having these kinds of thought when you're just 18, maybe it's too early. But i want someone who will care for me, who will love me. I feel so lonely, forcing a smile on my face, trying to live my life. but this ache never fades, it only grows by the day. sometimes so powerful, i can't seem to breathe.. Why do i feel this way??