For starters, the way the word apathy is formed according to etymonline gives a hint at what the antonym should be:
apathy (n.) Look up apathy at Dictionary.com
c.1600, "freedom from suffering," from French apathie (16c.), from Latin apathia, from Greek apatheia "freedom from suffering, impassability, want of sensation," from apathes "without feeling, without suffering or having suffered," from a- "without" (see a- (3)) + pathos "emotion, feeling, suffering" (see pathos). Originally a positive quality; sense of "indolence of mind, indifference to what should excite" is from c.1733.
So if apathy describes the absence of pathos, then pathos is the opposite...
Alas, pathos, and certainly the adjective pathetic, have acquired quite a negative connotation, and it will not be generally understood any more as feeling, passionate.
So for words that do somehow, in a general way, describe that somebody is actively feeling about things around them, I would suggest passionate as an option:
having, showing, or expressing strong emotions or beliefs.
It seems interesting, when we look at the origin of passion, again on etymonline), that pathos has moved from general (if sometimes suffering) emotional engagement towards a negative, mainly suffering meaning in English, whereas passion, originally mainly meaning suffering, has moved towards a more general, and certainly nowadays positive meaning: I don;t think that most speakers of English will associate someone's passion for human rights with the passion in the passion of Christ.