What makes a smile look ugly?

[Shrek2OnDvD]

I can think of an ugly smile being something like having missing/crooked teeth and visible gums, but what actually makes a smile ugly and unattractive?

 

[LTNUser]

Ugly face

 

[Everything]

Bad haircut of course and terrible personality

 

[Angutoid]

Narrow palate is the main issue in both of these pics