Smiling often works as "insta-bonding", especially if you're a female. Because we're social animals - we need to know whom we can trust - who will cooperate with us, and who can harm us and we should avoid. A smile is a simple way to show just that - it's a signal of well being but also a message similar to "reaching out" to others, a sign of empathy.
That's why smiling in social interaction hold so much value and we definitely like to be around other happy people who also like us.
But the bonuses to smiling are not ending there, the amount of other benefits to smiling is almost overwhelming - there's a correlation between people who smile with many almost unrelated benefits, people who smile more seem to:
I. Live longer
II. Perform better and think more broadly
III. Will actually feel better if they're down (one of the cases where your body language affect how you feel)
IX. Will appear more successful and attractive
X. Will be judged more softly (the apologetic smile, similar to the chimp incident we talked about)
XI. Will get away with "it" more often - can you imagine hitting someone who's smiling at you?