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 :-)