For many of us, we lose touch with the primal, "primitive" faith of childhood due to disappointments/baggage from the past. Some might just call it getting more sophisticated, growing up.
A reporter once described the Lubavitcher Rebbe as having the "faith of a child." One unfamiliar with the Rebbe might assume him to be very simple, unsophisticated--how "intelligent" can the faith of a child be? But that is one of the countless things that are unique about the Rebbe: He had a 
brilliant, extremely deep, sophisticated understanding of Torah and vast worldly topics, endured many hardships, and yet managed to maintain a constant, pure, untainted faith in G-d.
So how do we stop our past-not to mention our present-from tainting our faith in G-d? By constantly increasing in our learning and mitzvot,* which taps into the strength of our G-dly soul, which forever contains the faith of a child.
*Judaism teaches that it is action that leads to faith.