It reminds me why I chose Dauntless in the first place: not because they are perfect, but because they are alive. Because they are free.

This quote is referring to the fact that although the Dauntless faction in the book series is not perfect, they still have certain qualities that the speaker admires. These qualities are that they are “alive” and “free”. Being “alive” could refer to the fact that they have passionate and curious minds, as well as a willingness to take risks. Being “free” could refer to a number of things, such as living without fear and having the courage to stand up for what they believe in. The speaker chose the Dauntless faction because of these two qualities that are not always found in other factions, and it is these qualities that remind them of why they chose them in the first place.