I am not so good in explaining Polish grammar, (and it was long ago when I was learning it), so let the Polish Language Council speak instead of me: ;) The only cases in which you have to use longer (stressed) pronouns:
In the beginning of the sentence ( "Ciebie kocham" - "It is you whom I love")
In the end of the sentence when it has to be stressed that this person is the real receiver of the action ("Kocham ciebie" when someone implies that someone else is the real object of affection ;)), or in the middle, but followed by an explanatory sentence ("Kocham ciebie, a nie ją" - "I love you, not her").
When the pronoun is the only word in a sentence - "Kogo kochasz?" "Ciebie".
I hope it is more clear now.
(Sometimes it is hard to explain rules, that you know since your childhood)
Ok, I know the proper answer is "Kocham cię" but when replying, why polish people use "Ja ciebie też" instead of "Ja cię też"?? It's like for example if somebody says "Jest mi zimno", the reply would be "Mi też" so it follows the same case... Kinda confused with this "Ja ciebie też" XD
It has to do with emphasis. In kocham cię the emphasis is on the word kocham ("I LOVE you"). Ciebie is an emphasized version of cię. "Kocham ciebie" would mean "I love YOU" or "it's you I love". So when answering, it seems to be more common to say "and I (love) YOU" instead of "I (LOVE) you".