Translation:Nothing is more important than friendship.
That's actually a twisted version of the actual quote, "The blood of the covenant is thicker than the water of the womb." Which actually has the exact opposite meaning of what people think it means. (The bonds you form by choice are more important than the ones you're forced into by birth. This especially holds true if you're from an abusive household, which makes sense when you consider the fact that many abusive parents like to use the twisted version far too often.)
"actually has the exact opposite meaning of what people think it means"
When one person says a phrase to another, and the latter gets the same idea which the former wanted to pass - that idea is probably the meaning of the phrase. That's how the language works.
People who use that 'blood is thickier than water' (and, as I said, a surprisingly similar idiom of the folk origin in Russian) - they give it a meaning, not dictionaries of any kind. Any "actually has the exact opposite meaning of what people think it means" is wrong, because everything means what people think it means when it comes to the language.
Yeah, one more shot in that neverending prescriptivism vs descriptivism holy war. I guess it's funny when these things are said in a broken English of a foreigner.
Although, I agree with you on the rest of your message completely. The bonds we choose to have are stronger and, well, better for us.
'Nada' always means 'nothing', it's only used in negative constructions. Some of these constructions (those who have the double negative) happen to have word 'anything' when translated to English.
"No pude hacer nada" may be translated as "I couldn't do anything", but the actual meaning is "there was nothing I could do". Nada = nothing.
The rule is still there, but it only affects nouns that begin with a tonic a syllable sound. For instance: agua, águila, ala, hada all use el, but amistad, abeja, alacena, agüita (diminutive of agua) all use la because the first a sound is not tonic. Also, notice that this rule only applies to nouns, not adjectives, and there are a few exceptions.