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it seems to me that you are a positive person, which is why you can smile when you are in a lonely place. when you are with those people that you converse with, they make you more comfortable, and you can smile. you are not bottling anything up.
on a very deep level, maybe you are bottling things up, but that requires a lot of effort. it takes way less effort to smile and be merry with friends. and I believe we'd all rather do that, so we don't make an effort to combine our loneliness with our pleasure. I don't think it's a zero-sum game. I think you have two different dynamics at play in your conversations, so you may harbor the loneliness, but you don't have to battle it when you are with your friends. comfort easily brings your happiness back into play. I believe you ignore or forget about that loneliness, but you don't fight it constantly. you can have both emotions at the same time.
I'm sure some people do bottle their sorrow up. but that's not our responsibility. I smile as much as I can, especially at people I pass on the street. unfortunately, in our society, people have stopped making eye contact and smiling at each other.
I think it comes down to comfort. most people are not comfortable smiling at strangers, so they don't. you are comfortable with those you converse with, and they with you, so the smiles come out.
I am also very curious about the social world. I am speaking from experience and textbooks alike. this reply was fun for me, rarely do I get the opportunity to approach a situation like this as a neutral party. but everybody's different. so I may be completely off. but I can tell you right now:

I smile because I am truly happy.

-Mel

I thank you for the well thought out response. Fortunately, my dark mood seems to have passed for the most part. I do attempt to be positive, even when things look pretty bleak. Doesn't always work, but I try.