It is a problem for the newbies getting merits, the eagerness is endless as they anticipate merit in any content they have put up effort with.
This is true for newbies who has been here before and coming back with another account. For someone getting to know about the merit system will not be so eager to earn it.
But it seem not coming. Is it that those who have excess of merits reserves their generosity or there are laid down principles in writing a Meritorious contents.
Strictly none of the above. You may see one with much merits and at that particular time has no single smerits to share. Excess merits as you said doesn't mean there is always smerits except for merit sources. And it seems that merit sources run out of smerits sometimes.
There are no laid down principles in writing merit worthy posts.
You get merit when you make someone know new things, you help solve people's problems or answer questions. So, see merit as a reward for doing good. From henceforth you can consider to be helping people.