What are the most common reasons people don't dps?

[Sebazy]

Healing are the more situational role. That's because, while tanks and DPS must mainly deal with a highly-scripted AI, healers must deal directly with humans to fill his role.

I do agree with the core of your point and yes, a healer's overall performance is in many regards, restricted by the consistency of their group. Don't get me wrong, an Ice mage doesn't stop me DPSing, neither does a flash spamming PLD (It is entirely possible to hold rock solid agro on PLD whilst doing very negligible damage).

However, I will correct you a little on your point that I quoted above. First off, at the end of the day, incoming damage is still a part of that very same script. You can see the damage coming and you've usually got a good amount of time to prepare and adjust for it in casual content providing you are playing with your eyes open.

Secondly and perhaps more importantly, triage! If there's one thing my experience has taught me well, it's the art of recognising how much healing is needed and more importantly, where it's needed in any given situation. If you're blasting multiple Cure IIs at that 1k DPS Ice Mage then you are literally wasting your time for the most part.

I prefer to simply throw a regen at them and leave them to it, that's 21 seconds where I don't have to baby sit them in most dungeons and for the most part, that's plenty enough to keep them going. In situations where it isn't, that's because they've just got themselves one shot and nothing you could do was going to save them from that anyway. This holds true for a surprising amount of mainstream content and it's usually only those occasional moments where a boss is about to do an unavoidable AoE right after a painful mechanic.

TLDR: Don't take someone else's mistakes and make them your own.

EDIT: Notice that I am not speaking that low-effort healers shouldn't be putting some effort. What I'm saying is "stop with that war against healers or let's demand maximum effort for everyone. Pick one".

Doing a modest amount of damage as a healer absolutely isn't maximum effort in any stretch of the imagination. Maintain your dots and the vast majority of people will be happy with that, it's not until you hit savage and beyond that people start expecting more than that.