Simon17 gave a very good summary of the reasons.
1) Most honestly believe they are serving God in the only correct manner
Very true for most (but not all) JW's. About the purest motive you could list, but probably the weakest one as well.
2) (for the R&F) The feeling of belonging to something specialWe hear about this all the time from the platform, because it's a known motivator: the "united international brotherhood" that only we, as a special group, are part of. This feeling of being part of an "inner circle" within society as a whole is very powerful. For those who are socially disadvantaged (broken families, nomadic upbringings, or pretty much any born-in JW), having a prepackaged set of 'friends' lovebombed to you upon baptism
3) (for the higher ups) The feeling of power/authority/respect and influencePeople can get drunk off having Power, Recognition and Influence, however petty (or significant) it actually is (remember Hall Monitors in school?) In an environment where cleaning the toilets is a privilege, any smidgen of authority becomes incredibly potent.
4) (For the higher ups) Lack of real monetary worry...Add to this a level of luxury not readily visible to the rank-and-file publisher. Rutherford's excesses are well-known, what with his depression-era Cadillacs, or Beth-Sairim - but even now, who among us really knows how the higher-ups live and travel? Want to visit Europe? I've never been, but I'm sure any one of them can appear at any Convention or visit any Branch they want, and it's a done deal. I doubt those at the helm sleep two-to-a-room in cramped dorm-style rooms - their living standards likely surpass 90% of the 7+ million members they represent.
They proudly announce there are no salaries, true; but as any HR rep will tell you, "a salary is only one part of your Total Compensation."