"I love cats."
Translation:Εγώ αγαπάω τις γάτες.
I don't know why, but you definitely need it. "Εγώ αγαπάω γάτες" is very awkward and unnatural, and if someone used it, they would mean that they love some cats. Τις here means all of the cats, so cats in general.
The same structure (definite article in plural + plural noun) is used with all verbs that express feelings with nouns that are countable in English (I can't think of an exception at the moment):
- Μου αρέσουν τα βιβλία = I like books
- Φοβάμαι τα λιοντάρια = I fear lions
- Μισώ τους υποκριτές = I hate hypocrites
In the case of abstract ideas, we use definite article in singular + singular noun with the same meaning:
- Αγαπώ τη ζωή = I love life
- Μισώ τον πόλεμο = I hate war
- Δε(ν) μου αρέσει η θεωρία = I don't like theory