I think being casually apathetic started in high school. Not really caring about others -- in a school context, in particular. I don't recall caring about bettering myself either. I think anti-depression drugs had a lot to do with that, looking back. Let's be honest: Zoloft, Effexor, etc have one goal: to even out the serotonin in your brain enough so you can function. They can and will make you numb as well. Just a disclaimer. Part of the process.
I probably could've been a better wife the first time around, as a result. I will continue dealing with that. I'm sorry that I wasn't mature enough to deal with what was brought to the table: from either end.
My point: you understand what has happened in life and you move forward. You learn what your body chemistry is so you can tell your kids what they might have to deal with. You figure out if you will be a good mom based on your experiences.
I started caring again because of love, "real love" and coming to the adult part of your journey in life. I'm just glad it was when I was 26 or so, not later on.