The old saying is that 'Free stuff isn't worth anything,' meaning that things you get without working for aren't valued. Free lunches for schoolkids are thrown away — the kids don't pay for it. Government handouts are wasted and scorned as 'not enough.'
So working for things makes you value and appreciate them, and, consequently, stick with them. A club that takes a lot of money and/or effort to join, all things being equal, is a club you're more likely to remain in than one that's relatively easy to join. This also applies to things unlike clubs. If it's hard to get into a college, said college retains students more consistently than colleges with easy entry. Anyhow, there seems to be an overarching principle behind all this — effort tends to commit. And any business, church, or whatever that inspires commitment is more likely to be successful. This applies to things like eHarmony and hard-core fundamentalist churches. The higher the bar for entry, the more successful the organization is. This is gone into in more depth on the very nice blog of Satoshi Kanazawa HERE.