I like apples - correct...why? I like pizza (not pizzas) - why?

Many foodstuffs are considered uncountable when prepared and countable in the field or on the farm:

I like apples, chickens, and tomatoes on the farm.
I like apple pie (baked apples still look like an apple), roast chicken, and pizza with tomato and pepperoni in the restaurant.

Pizza, spaghetti, stew, ham and such dishes don't look much like any countable object anymore, so are considered in the uncountable form.

We normally use the plural of countables because it is not any one of them that we like, but the whole set of delicious cookies or cuddly puppies. The singular is possible when the concept becomes more intellectualized: 'I like the tiger: it is a mysterious animal'. You can do the same with 'the dog', but 'I like the dog: it is an intelligent beast' is a rather abnormally objectified attitude'.

Hope this helps. Others may have more comments.

Sorry, Paco-- I'm not really out to get you, but the situation you have set up would require 'I would like an apple'.

I agree that we wouldn't say 'I would like (some) apples', because that sounds greedy indeed. However, this is a request, a different case from stating likes and dislikes.

In the request, we are considering how many of a specific item we want: 'I'd like an apple/three apples'.

In stating likes and dislikes, it is a matter of giving one's opinion of the type of item: 'I like/don't like apples'-- as a type of fruit, or 'I like/don't like pizza'-- as a type of breadstuff.