'Serious' Demeanor for a guy?

Gilrs, do you generally not notice, ignore, avoid etc. guys that seem serious? My friends often tell me I need to seem more approachable and smile more often. For some reason I am usually quite straight-faced and monotone and I feel like this might be one of the reasons why I don't have any girlfriends or friends that are girls.
'Serious' Demeanor for a guy?
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