Using article in a sentence

Question

I am always confused about using articles. I have three sentences.

1. Depth of friendship doesn't depend on length of acquaintance.
2. A passion for pleasure is the secret of remaining young.
3. Friendship is love with understanding.

1). Which one is right, depth, or the depth? (length or the length).
2). Passion and secret have articles but not pleasure. Is there any rule for using articles in this sentence?
3) Why doesn't friendship have an article?

Answer 1

(1) and (3) refer to friendship as a concept rather than a particular friendship between two individuals. You could instead say 'The depth of a friendship...'

Similarly, (2) refers to pleasure in general rather than one particular pleasure.