It's good to smile at people when you see them in the street. But, not everyone is deserving of a smile. Some people are mean and nasty and really don't deserve to be smiled at.
If someones smileable, you consider them worthy of your smile, and are willing to offer up a free smile, just to be nice.
Note: Not to be mistaken for 'Smileability'
Did you see that guy? He was totally smileable
! I'd smile for him any time. What an awesome dude.
Wow! She's really smileable
, she's just so nice. I could smile at her all day long :-)