In Of Mice and Men, how is Curley lonely?

Curley can be seen as lonely because he is unable to emotionally connect with anyone on the ranch.

Curley brings much of his isolation on himself.  He is threatened by "big guys" and demonstrates physical aggression often, channeling his previous occupation as a boxer. The ranch hands know this, as seen in how Candy describes Curley as always wanting to "pick on big guys."  Curley is also alone because of his status as the boss's...

Curley can be seen as lonely because he is unable to emotionally connect with anyone on the ranch.

Curley brings much of his isolation on himself.  He is threatened by "big guys" and demonstrates physical aggression often, channeling his previous occupation as a boxer. The ranch hands know this, as seen in how Candy describes Curley as always wanting to "pick on big guys."  Curley is also alone because of his status as the boss's son.  No ranch hand really wants to get to know Curley because they know that it might impact their job on the ranch.  Curley accepts the social estrangement associated with being the boss's son and does not seek to overcome this for the purpose of forging emotional connection with the drifting farm hands who make up his social world.  

Curley is also lonely because of his relationship with his wife. He and his wife are not emotionally connected to one another.  As Whit says, Curley's wife spends her time looking for Curley, who, in turn, spends his time looking for her.  Neither one is able to find emotional comfort within the other. The result of this emotional separation is a feeling of loneliness within Curley.  There is a difference between being alone and being lonely.  The former indicates some level of contentment.  Curley is not really alone, but he's lonely because he is unable to find a sustainable emotional connection.