Tongue-in-cheek: "when their pathological need to do something outweighs their pathological need to do nothing."
In more detail: there are several different kinds of deep-rooted psychological needs that ambition might be powered by, and I think the resulting different kinds of ambition are different enough to discuss as distinct entities (in particular, they vary in how prosocial they are). Some possibilities off the top of my head, not mutually exclusive, inspired by Enneagram types:
1. Reinforce a particular identity / self-narrative (e.g. "I'm special" -> strive to become a celebrity or w/e, see Instagram influencers); Enneagram 4
2. Get people to like you (again see Instagram influencers); Enneagram 3
3. Have power over people (some politicians, maybe); Enneagram 8
4. Have fun to avoid feeling bad; Enneagram 7
(One way to probe this in a given ambitious person is to look at what coping mechanisms they turn to when they fail. E.g. if it's about reinforcing a given identity through some ambitious project, when that project fails do they start reinforcing that identity in other ways?)
Then there's genuine compassion, which is the cleanest power source for ambition I've found so far, and arguably the most prosocial (there might be others, e.g. childlike joy and wonder). I am quite concerned that most of the ambition in the rationality / EA space is not being powered by genuine compassion; personally, most of the time I've been here I've been powered by a combination of #1 and #2.
There are also several different kinds of deep-rooted psychological needs that lack of ambition might be powered by. Again, some possibilities off the top of my head, inspired by Enneagram types:
1. Not drawing criticism / pissing people off; Enneagram 2, Enneagram 3, or Enneagram 9
2. Avoiding the feeling of not knowing what to do; Enneagram 5, Enneagram 6
3. Sense that ambition is morally wrong / corrupting; Enneagram 1
4. Sense that ambition is not your place / not the sort of thing people like you are allowed to do; Enneagram 2, Enneagram 3, Enneagram 4
Historically I think a lot of my lack of ambition was powered by a combination of #1, #2, and #4, although it's hard to disentangle. There were also less psychological obstacles, e.g. I was tired all the time because I was eating, sleeping, and exercising poorly, and had an awful social life; real hard to be ambitious or agentic in that state.
To summarize, I mostly relate to ambition as a relatively surface-level psychological phenomenon that's being powered by deeper dynamics, and I think at least as much in terms of obstacles to ambition as in terms of ways to cultivate ambition.
Epistemic status: based on lots of personal development work and looking at other people's psychology and personal development, e.g. via circling; especially, noticing my own level of ambition increase drastically the more work I do on myself, and looking at what seem to be the gears of that.