how can faith be so easy when you're young but gets difficult when you grew older?

I remebered the day when I was really young and I used attend the session where it was held every Saturday during when mass would start and it would end just right after the homily. This was held so that children my age then would understand the gospel and homily better. It like cathecist during mass. well I remembered one time the facillitor asked all of us, "how could you believe when you cannot see?" I guess the gospel was about Simon didn't believe that Jesus has rissen from the dead. A boy about 12 then said, "we just believe. we didn't need to see. we just believe." and me then at the back of my head was thinking, "duh. we don't need to have to see. we believe already so why do we need to see?" my faith then was so great. Now I'm not so sure. I wish my faith was just as strong when I was 12. just when I needed it the most, it seems so far away.