My excuse is that I was born in the religion. But if someone wasn’t, why would they become one?
I am born-in too. What I know from the experts is that it's a combination of factors, including timing and circumstances of the person who joins, some of which apply to some people. Here's what some experts say:
Decision point - The cult arrives at a time when people are facing a turning point in their lives, and need support.
Dissatisfaction with state of something (the world, their lives, their community, etc.) - Certain events affect people and they want answers or an explanation for their suffering, and the cult appears to give them comfort and/or answers. The WT offers paradise forever on Earth.
Loneliness - Some people just want to belong to something. The WT offers a "spiritual paradise" and a congregation that is forced to tell that they are a cohesive unit where people love each other.
Loss - The cult offers hope and comfort from loss. In the case of the WT they offer people being able to see the loved ones who passed away.
Play on ego - Some people never get the opportunity to lead. The WT offers "privileges" of being a pioneer, a MO, an elder, etc. This one seems to speak to people who never had educational or career opportunities.
Promise of certainty; relieve doubt - They make promises that "all your problems will be solved". The WT talks about living in paradise where one will not even fear wild animals.
Spiritual search - Some people are actually looking to nurture their spirituality. The WT offers gazillions of publications providing gazillions of explanations about why the Bible is so clear.
The “buzz” / emotional arousal - The born-again effect is a feeling that some people like. It's called the "religious high". It took me some time to understand this one. I found a book with the title "When God Becomes a Drug" that explains well the effect of emotional and religious arousal in people.
Vicarious celebrity - People relate to the experiences that they hear others telling. The WT notoriously does that in their assemblies, always having people sharing their testimonies and experiences, always being some kind of horrible person who now isn't thanks to their Jehovah.
Attractiveness of leader - Some people are charmed by the leadership. In the WT this is typical of when people attend a talk from a very skillful speaker.
Escape from freedom - Some people have a sense of inadequacy about their own lives, and they feel that they need some kind of structure.
Health reasons - The cult promise comfort and and answer to their suffering, or hope about their conditions getting healed. The WT promises all that in their paradise.
Stop using drugs - Some people feel that being part of a religion or some kind of group that doesn't allow drugs will keep them safe, abstinent and sober. The WT promotes a "healthy lifestyle" in that respect.
Escape abusive families - Some people are in bad domestic situations (caused by them or others; remember that there are many current JWs who were/are violent abusers). People look for religion as a way of changing family dynamic, or as a way of sheltering themselves from abusive situations.
Relationships - Aside from a sense of community, some people believe that only good people join and stay in those groups, hence, they think that the groups are good places to find suitable partners to form families. The WT promotes marriage among them.
I see some of those things in my family. My mother joined because she was completely alone in Puerto Rico after moving from the Dominican Republic, and only had my violent wife-beater drunk father with her. In the JWs she found an explanation for her suffering, a congregation, something to do, hope for a better life, a ways to improve her level of literacy, and a promise of happiness and life forever.
My father joined in an attempt at stopping his violence, and to stop hanging out with his drinking buddies (to stop drinking). He also saw an opportunity to teach and lead others, something he likes doing but never had the chance to do again. Never mind that he stopped beating my mom and using beer as an excuse, but started beating us his children and using the Jehovah crap as an excuse.