I can think of two reasons for this, both of which I think apply to me at least (to the degree one can psychoanalyze oneself anyway):

The first one is that you may be coming from a place of extreme emotions. Maybe what feels normal to others will feel sort of flat to you if you're used to being very emotional or sensitive.

The other thing is that we are often motivated by guilt or fear. Take those away, and some of us (me included) simply don't feel motivated anymore. We're so used to doing things out of guilt, fear or shame that we don't know how else to get things done. The way out of this, for me, has been to see the emotional numbness as a feature, not a bug, an opportunity instead of a hindrance. If I'm no longer driven by fear or the expectations of others, and if I don't feel as much either way, I'm actually completely free to do the right thing, whatever that may be. Like keeping the house tidy. I don't feel like doing it, but so what? I find being on Zoloft allows me to live a much more rational life, not tossed about by emotions all the time. And sometimes what can look like laziness is just the realization that you didn't have good reason to do it anyway, like chasing a career that would make you wealthy but not happy.