It makes sense when you understand what an apostrophe is for. It is used in contractions (e.g., shouldn't, can't, and you'll) and for showing possession for a noun (e.g., Tom's diner, Bob's hardware, and Simba's tail). The key word there is noun. This does not apply to pronouns. Possessive pronouns NEVER, EVER use an apostrophe.
YourYour is a possessive pronouns, meaning that it is the pronoun "you" that owns something.
Your cats. Your homework. Your problems.
Notice the lack of the apostrophe. If you use the apostrophe you get another word and meaning. You get...
You'reYou're is a contraction for "you are."
The TestIf you are confused, the trick is to say "you are" to test if you got it right or not. If it works, then you use "you're", if it doesn't, then you use the word "your".
You are cats. You are homework. You are problems.
You are here. You are beautiful; you are not fat. You are late!
The first group does not make sense, so you use the word "your". The second group does make sense, so you use the word "you're".
Thanks everybody for visiting, you're awesome!