I have horrible luck with healers.
My 24 guard tank gets a queue for mando raiders sm, with 2 commando dps, and a sage healer. Seeing as one commando dps is quite well geared (almost has same HP as me, with his aim high as well) I put guard on him. As a guardian I find commando dps tough to compete with for aggro, due to their aoes and good single target. It makes sense to me to guard him over anyone else, due to threat reasons. Immediately, before we even pull the healer cries out for a guard. I tell him the dps is more likely to pull threat. He then says he will quit if he does not get a guard. I really dislike people who act like that, so I continue to try and reason with him rather than giving him guard. He drops group before first pull. He then sends me rude messages calling me "a fcking horrible tank," and "dumber than a pile of rocks" and tells me that guard does nothing about threat all it does if reflect damage to me. Before I could respond he ignores me. It's things like this that make me hate grouping.
That sounds like about 1/5 of the healers I encounter while doing lowbie FP's, sometimes 55s as well