Why is 你们爱猫 translated to "you love cats" (and not e.g. "you all love cats")?
http://www.cantonese.sheik.co.uk/dictionary/characters/5/
  
  們:
  
  an adjunct to a pronoun or noun to indicate plurality


Pronoun:

你 (you) is a singular pronoun in Chinese, Adding 們 after it make it a plural pronoun- 你們(you)

我 (I / me) is a singular pronoun in Chinese. Adding 們 after it, makes it a plural pronoun- 我們 (we / us)

他 (he / him) is a singular pronoun in Chinese. Adding 們 after it, makes it a plural pronoun- 他們 (they / them)

Noun:

The adjunct 們 is mostly used with human pronouns (e.g. - 你們, 我們, 他們).  Although it can be used with a noun, it is restricted to nouns for human only. 

For example: 


戰士 warrior / 戰士(們) warrior(s)
學生 student / 學生(們) student(s)
工人 worker / 工人(們) worker(s)
人 person ; people / 人們 (people) * 人 can be either singular or plural 


Using 們 with nouns is much less common and it can often be omitted,  because plurality can often be indicated by use of plural classifiers (e,g. - a group of; a class of) or plural adjective (e.g. - those; these).

們 is always used on people or other situations when you want to express personification. 

If we write 一群學生 (a group of students) or 那些學生 (those students) we no longer need 們 to indicate plurality. We know it is not a single student. 

Adding 們 and write 一群學生們 or 那些學生們 would be redundant.