Why is ammonia a Lewis base?
Well what is a
Is it not an electron pair
And of course ammonia CAN behave in this capacity...i.e. consider its Bronsted basicity..
The equilibrium lies to the left as we face the page. And ammonia is a potent ligand towards transitions metals...
And thus ammonia fits the description...still pretty smelly tho, and when you open the bottle the neighbours complain if you are a bit careless...