The personality trait conscientousness involves being hard working and doing things that aren't intristically interesting.

What motivates people to do that? Is the reward system active in highly conscientous people just as it is during pleasant things?

Maybe they find it intrinsically interesting. Maybe they find the task to be pleasant. You make far too many specious assumptions in your premises with these scores of threads of yours. 

I would say accomplishment is reward for those.

6 hours ago, Hans de Vries said:

conscientousness involves being hard working and doing things that aren't intristically interesting.

Citation?

4 hours ago, iNow said:

You make far too many specious assumptions in your premises with these scores of threads of yours. 

I dislike the use of broad generalizations while simultaneously trying to draw shallow conclusions from guesswork about people's motivations. We don't have a specific rule against it, other than to keep asking for evidence and support, but the other side of this particular coin is a lack of engagement once the thread is open.