Loneliness.

James had always been lonely. He found making friends was a difficult thing to do, being rather shy and anxious in nature. This wasn't helped by the fact his mother and father were constantly on the move as part of their job, so only in the last two year had he settled down. He had turned 18 on the very first day of the school year, so was able to live alone as he worked.

The word was "alone". He was so awfully lonely, no friends, nothing to do with his family...He just attended class and worked, and on his days off he would lay alone in bed or sit on the sofa and watch TV in his small, lonely apartment. Just him, and nobody else. He hated this, but was scared to make friends. He was picked on so much in the past, and his mother and father had both told him he would never be wanted by anyone.

Y/C is more popular than mine is. She has lots of people whom call her a friend and hang around her. She could have her pick of anyone she wanted, and is liked by everyone, so it seems. Yet she too is lonely. Surrounded by people, yet not one seems to be a true friend nor care all that much for her other than to get to the top of the ladder.

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So I figured your character and mine are paired together for a while as part of a school initiative to make the less.. popular kids less lonesome and try to prevent bullying. A good idea not executed greatly, but that's how it was. Yet, they actually hit it off well. They find they have a lot of interests in common, share the same sense of humour and so on.

As such, they become very fast friends and very close in a short period of time. As close as they are, they both fear the other will leave and this prevents them from becoming as close as they want to.. no, need to be.