"Your neighbor is noisy."
Translation:Dein Nachbar ist laut.
For the same reason that "Yours neighbour is noisy." is incorrect: you need a possessive determiner here (before a noun), not a possessive pronoun (which stands instead of a noun).
In English, you can say "Your neighbour is noisy" (with noun) or "Yours is noisy" (without noun), but not "Yours neighbour".
In German, the possessive determiners inflect like ein or kein, and so they have no gender/number/case ending for masculine nominative singular, neuter nominative singular, or neuter accusative singular.
Thus for Nachbar in this sentence (masculine nominative singular) it is euer Nachbar with no ending on the basic form euer-.
If you left out the noun, you could say Eurer ist laut. (Or Euerer ist laut. if you wish -- also correct, according to the standard, though it's much less common in my experience and actually sounds wrong to me.)
No idea. That's just how it is in German.
Possessive determiners (before a noun) inflect like kein or ein and don't have an ending in masculine nominative singular, neuter nominative singular, or neuter accusative singular (Mein Vater und mein Kind sehen dein Kind).