We each have our own individual personality, and it's true that some people are harder to get along with than others -- sometimes much harder! No wonder the Bible urges us to "clothe yourselves with compassion, kindness, humility, gentleness and patience" (Colossians 3:12).
Why are some people like this? Sometimes it's because of pride; they think they're better than anyone else, and this keeps them from being friendly. Sometimes it may be for psychological reasons they may not even realize. Someone, for example, who's been hurt by others or was abused as a child often will be angry or suspicious as a result. And sometimes it's because a person simply wants to be left alone, for whatever reason.
The real question, however, is this: How should we react to people who aren't easy to get along with? It's easy to like people who like us, but what about those who are hard to like? Let Jesus Christ be your example. Instead of lashing out at those who hated Him, He still loved them and was even willing to give His life for them. The Bible says, "If your enemy is hungry, give him food to eat; if he is thirsty, give him water to drink" (Proverbs 25:21).
Pray for your cousin, and ask Christ to help you love him just as God loves him. Then do what you can to be a friend to him and point him to Christ, even if this is difficult. Your patience and love could make the difference.