He didn't have a lot of kids to play with when he was little. It wasn't because he was a shy boy, or because no one liked him. It was because he stayed at home with his mother, with no siblings or cousins.
When he was six he asked his parent why he didn't have a younger brother or sister.
"Where too old to have more children," his parents had explained.
So when he reached Hogwarts and made friends, he couldn't believe how nice it was to have friends. As the years went by, he grew more and more attached to his friends, until they became not just his friends, but also his siblings.