However, I think the term "campaign" came from the wargaming hobby that preceded D&D. And a military campaign is a large scale, long term series of interrelated conflicts that involve many squads and platoons.
Regardless of it's origination, I've always approached the term "campaign" with the idea that it incorporates the adventures of many different characters and parties over a period of time in the same setting. Hence the term, campaign setting. That is, a campaign is much more than a single character or group of characters, it encompasses whatever happens within the setting, although usually loosely connected. This is often through PCs, NPCs, a location and a long-running series of events (such as a war), etc. that ties things together. In my case in the Forgotten Realms, even though I've made the 4e 100-year-jump, and the players that were around in 1358 DR are no longer even living in the area, their characters and activities still factor into the group today. There are a few connecting points as well, as new players have come in and old ones have moved on.
So for me, by that definition, all character motivations help build the campaign, because new stories start when old stories end (or move in a non-adventuring direction). The setting grows, and grows more familiar, because NPCs are now ex-PCs that the players already know. Ex-PCs can become PCs again when a new motivation arises during the course of the campaign.