Why my girlfriend always holds my arm?

Always when we walk together she holds my arm. Even when we are sitting at a table she prefers to sit near me and hold my arm or put her head on my shoulder.
The things get worst when there is another girls like her friends are with us he stick to me like I can't move!
I don't say don't love it but think its too much especially at my age I'm 27 and think this is a little childish she's 20 ant think its sweet and shows love.
Why my girlfriend always holds my arm?
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