Your smile will give you a positive countenance that will make people feel comfortable around you. If your looking for a job and your one of the top two best candidates and one who takes the time to smile and everything else is equal - guess who gets the job?
When you smile it makes people smile back and it makes people feel welcome, invited and happy. When you make people feel this way doors open up to you.
Open up more doors by smiling and be happier yourself.