Communication. Just communicate. With most tanks and groups I play with a lot, I'm a very offensive-minded healer, and it works really well because the tank knows that and manages cooldowns really nicely. I even do it with a couple tanking friends wearing their all STR gear. Move over to DF or PF and who knows how anyone is going to play, what gear they have or how (or sadly if) they might use their defensive cooldowns. It's no wonder healers back off and simply do not dps. Just be sure people know that ignoring mechanics is what you wish to do so they can adjust accordingly.