There are two forces on the Island. The first is a healing force. It brings people with troubled lives to the Island and helps them reconcile with their past. The second force is fate which tries to kill everyone who should be dead (they did fall from a plane after all).

Pros

This explains why the Island appears sometimes to be good (they survived the crash; Locke can walk; Rose is cured; Charlie kicked drugs; Sun is pregnant; many are finding purpose and peace) and sometimes evil (the monster; the Others; all the people who died).

In some cases the healing force seems to preserve people until they have reconciled with their past, at which point they are killed (e.g., Charlie and maybe others, such as Eko). But this doesn't always seem to fit (Paolo).

Desmund's visions, the things he is told about fate, and the inevitability of Charlie's death all show the force and inevitability of fate once someone is set to die.

From this angle, both healing and fate would conspire to keep people on the Island and out of touch with the outside. Healing needs time to work, while fate will not allow people to escape.

Cons

This doesn't explain their connected pasts, or the numbers. It's unclear how this applies to the people on the Island before the crash. The healing force seems to have begun by killing people (in the crash), which is a strange way to heal.