I'd say my "reason" for it varies as well. It's like an on/off switch, sometimes it turns emotions on, other times off. And sometimes it works well, sometimes it does not, and it's not that related to severity I find.

I've never thought of it as "if your hurt me I can hurt you", simply because I've never felt that anyone (besides my T) cared enough that it'd hurt them. I do think it would be useful to bring up with your T though. I doubt your T would get hurt over that, it sounds quite logical to me, even if "normal" people might think it's manipulative.