I don't think it's necessarily an either-or situation. There's probably a lot of factors that go in to someone deciding they're anointed - I suspect a lot of it has to do with who they've seen partaking (if they're anointed, surely I must be) or with a desire to be anointed (either to rule in heaven or for the prestige now) or other environmental factors. They take that and eventually rationalize their way to thinking they're anointed. I would guess that anyone who's partaking probably genuinely believes they're anointed, but that doesn't mean that the process that got them there was altruistic. People can convince themselves of all sorts of things just because it would be convenient if they are true.
One thing that seems to be true in my limited experience with partakers - they all seem to have a pretty high opinion of themselves. I guess you'd have to - JWs already think they're the best 0.1% of the population. To think you're anointed, you have to believe you're the top 0.1% of that.