My guess is that some may enjoy the challenge of trying to break something and enjoy the destructive aspects to testing that can be interesting. Picture something like, "See, if I do this, the whole thing goes 'ka-bluie!'" where the end result is that something that takes down the mission critical system is caught. Others may enjoy the pride that they have in ensuring that poor software doesn't pass through Quality Assurance that may be a source of motivation.
For growth opportunities, I could see there being the management route of becoming a team leader, test group manager and then director for one line. Another line could be to get into other departments like becoming a developer, project manager, or business analyst for a different route.
Developers and testers can share some opportunities though I would note that the tester may not be seen as being as technical as the developer. Thus, architects and other deep technical positions may be more likely to go to developers rather than testers, would be my opinion on it. I don't have facts to support it but there can be some snobbiness between testers and developers that shouldn't be lost here.
Managing expectations and understanding how far each side should go in communicating problems is likely a key point on your last question. There can be the concern that minor bugs are acceptable in the first version of software which may anger the tester so this is something to understand in some ways. Similarly, a major showstopper bug should be handled before release would be the flip side as something to note and be prepared to defend one's position as well as understand that someone else may overrule your call.
In response to yoosiba's comments:
Some people may initially start out as a tester and progress into being a developer. Would you not concede that those wanting to start in IT may start at testing or help desk roles? I state that some testers may move into other roles and state at least a couple of other possibilities, such as project management and business analysis that I would hope are also not exactly straightforward progressions either.
Some testers can enjoy stirring the pot, though these can be rare individuals at times. There are some people that enjoy breaking things or to put it another way stress testing things to see what happens if we try this or that.