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

[Cynfael]

There's only 1 healer in each party before lv 50, you can't learn from looking at other player to that point if you don't play something else.

I'm going to call this out as an excellent point because it applies to all sides of the discussion. When I was a new player, I didn't start paying much attention to my DPS contribution in group play until I hit lvl 50 and began to take note of how (then) end-game content flowed and what other healers were doing. Once it really hit me how choreographed the action really was and how excessive healing did nothing except add enmity and make one appear to be busy, I made adjustments and began to play better with others.

As for the rest of it, remember not to confuse the much-maligned "lazy healer" with new or inexperienced healers. As many posters have qualified (innumerable times), players who are learning are not considered to be the problem. It's widely accepted around here that if they are given the opportunity and a little patience, they will generally improve. The players who refuse to contribute out of some misguided principle, who refuse to see the value in making the effort, who feel entitled not to do anything beyond healing, or who truly are just that lazy are the actual problem.