Most people either stay with their religion of origin (the religion they were brought up in) or become agnostic (do not follow a religion). For those people for whom religion is of great importance they may switch religions because of theological differences with their current religion. In most cases where people convert, this is due to social reasons such as marriage or other changes in family arrangement. Changing faith for reasons other than personal convictions is known as secondary conversion. Some religions welcome newcomers for whatever reasons whereas other discourage secondary conversion.
People who change religion due to conviction of faith often due so because their current religion does not fulfill their theological needs. Perhaps it is contrary to aspects of their lifestyle etc. Most such conversions are usually between denominations rather than between actual religions, although these major jumps do happen.
Electrostatic Attraction is the force that attracts one ion to another.
To my knowledge ex-jehovah's Witnesses joint a multitude of other religions. Some choose not to joint another religion at all. I don't believe there is one specific religion that attracts more former Jehovah's Witnesses than another.
Gravity
proximity and valence electrons
I love my religion, Islam, because it teaches people should love one another.
One person's superstition is another person's religion.
Colors and scents.
Changing from one religion to another is called the same for any of them: conversion.
There is no absolute proof for or against religions. Until such proof emerges each have equal validity. This is of course no bar to people that have a faith in a particular religion.
that we have to be helpful to one another
Almost all countries that have freedom of religion have much lower violence rates than countries that don't have full freedom of religion, because members of religions are protected from other citizens who don't like their religion as well as being protected from their own governments. People learn tolerance for one another if they are not allowed to harm one another. People are happier and more productive in life if they are not in fear of being persecuted because of their religion.
You don't. People aren't just inherently one religion or another. You choose it... you have to be converted and believe it, not just be born into it.