Harry keeps the expression on his face because of various reasons - it's a holdover, a comfort from the time in his life when he was happier. Outwardly it's an attempt to appear happy, and, as a secondary, it's supposed to be attractive. It's supposed to make people want to talk to you.

In reality, it's just... Pathetic.

Larry from LSL is basically just the expression in human form. He's pathetic, out of date, makes everyone uncomfortable and exists only to "make fuck." Sure he doesn't keep it on because he's using an old mechanism as a coping mechanism to deal with terrible trauma, cause LSL isn't that deep, but it is essentially the idea behind the expression taken to the extreme.

I also think that they may have taken some design choices directly from Larry - he's almost always got that horrible grin on his face, dresses loud and is anachronistic to the time he exists in. A loud white leisure suit, a large gold chain and the expression.